Loading Open Wire Lines

Loading: Open Wire Versus Cable Applications

Open Wire Loading Coils Antique Picture

Rare photo of storm-guyed structure sporting top arm loading coils (black cases). Note the use of break-irons to dead-end the loop pairs to each metal-cased coil assembly.  Original coils were placed in wooden cases and surrounded by air, unlike these above, whose internal parts were oil insulated and cooled.  Wood was thought to mitigate the stray magnetic effects leading to possible energy losses.  Photo date: shortly after World War I or early 1920s (note automobile style in background).  Photo source and location unknown.


Many of you have seen the very large cases or “tanks” attached to a pole on a cable route.  These are separated from each other by considerable distance.  At inspection, the ground viewer will see an exposed cable stub leading from the main aerial lead- or suspended plastic-sheathed cable spliced into the pigtail of the tank-like device.  These are “loading coils” and are of various sizes.  Some, as were placed on the “A&B” cable between Chicago and Omaha on the transcontinental route, were the size of 50- and 100-kVA electric pole transformers.  Their cases were mounted on a two-pole structure with two steel “I” beams between poles.  The earlier models were square; the later 1920s and after, round.  They could be as small as a couple of inches in diameter and a half foot long.

Loading coils were not placed in series with cable pairs for decoration or to reduce the power to customers.  Instead, they were “induction devices,” as regular A. C. distribution transformers, but did several things in regards to telephone D. C. circuit health:

  1. Cable circuits undergo passage through a loading coil which causes a “bandpass filter” effect.
  2. Each installation increases series resistance and inductance of the conductors (pairs).
  3. Placing loading coils into cable circuits increases inductance, thus reduces the speed of voice and carrier currents.  In fact, for voice frequency circuits, loading is applied liberally.  Unfortunately, for carrier, it deteriorates signals where they operate higher than the cutoff frequency of the loading design.

Now, why talk about cable instead of open wire, for introducing this topic?  The principles of loading on telecom voice frequency circuits are well understood today.  However, the only load coils seen today are those on cable–aerial, buried or underground.  After the mid-1920s, loading was removed from open wire even though initially applied in the early days on some long routes.  Today, cables are frequently loaded in many situations.  However, the effects of an inductance device such as a load coil on open wire circuits opened significant “cans of worms,” to their efficient operation and evolution.  So, while we see multi-pair copper cable lines loaded (POTS), to open wire it is a “No, No!”

Let’s go back in history and review why open wire saw such initial promise with these coils but was quickly removed, and instead, became the standard of quality cable pair operation.

Let’s “Load” Up on the Basics of Attenuation

What is “attenuation?”  This, in general, is a five dollar word for interference.  Interference on open wire is much less than when confined within a sheath as cable pairs.  Cables were instrumental, even in the early days, of allowing open wire signals to be transmitted into central offices (C.O.s) and various repeater huts.  Railways, their civilian counterparts in telecom, and other transportation organizations, used cables because they were less exposed to the elements, allowed use of limited clearance and space, and could be buried or placed in underground conduits.  Clearly, the cable had a major role to play not just into the future, but even in the early days of open wire applications.

Open wire toll plant, by 1880, had expanded to nearly 50 miles, while its rival, cable in 1883, had expanded to over one quarter mile in Boston.  This major initiative was surpassed swiftly by a ten mile cable route between New York and Newark in 1902. 

Open wire was rushing west, as an 1893 open wire toll lead furnished daily service between Chicago and New York.  This nine-hundred mile link was groundbreaking, but heartbreaking as well, since this appeared to be the longest stretch of line for which speech satisfactorily could extend  without impacting fatal electrical losses.

With the application of U. S. Patent 0-652-230, filed on December 14, 1899, by Mihajlo. I. Pupin, the loading coil became practical, however, Pupin only refined the thinking of an earlier scientist, Oliver Heaviside.  Heaviside was a brilliant mathematician who advanced calculus to analyze circuit elements.  By examining telegraph cables in use, he found there could be ways of increasing inductance, thus emphasizing further efficiencies of operation.

Heaviside sought a “distortionless” transmission line, but found this could only occur if several factors combined.  These, he referred to as, “The Heaviside Condition.”  Series impedence (Z), must be proportional to the shunt admittance (Y), within all frequencies. No transmission line at that time was characterized by such a perfect situation, because insulators leak (low value) whether they are glass or plastic (in cables).  To remedy this situation, Heaviside considered improving inductance by:

  • wide spacing of each line wire
  • impregnating the insulators with iron dust 

These were rather impractical during the late 1880s, so Heaviside proposed placing spaced inductance devices along the transmission line at regular intervals.  This was a remarkable idea, but the powers that be in Britain, didn’t warm to the idea.  Because his ideas resided in theory, rather than a practical patented device, his suggestions were politely ignored.

Around the mid-1890s, John S. Stone, an AT&T engineer offered the idea of “continuous loading” analogous to Heaviside’s theories on cable induction, but it never saw direct application.  Later elements of his thinking found derivation in other forms of cable manufacture and design.   

Where open wire might have stalemated, loading furnished a load of relief to telephone engineers, who now might extend the western toll routes.   Soon followed some record achievements following the invention of the loading coil in 1900:

  1. Completion of the Boston-Washington, D. C. underground cable (455 miles) in 1913.
  2. Boston to San Francisco open wire lead (3,650 miles) in 1915.

As the H.M.S. Titanic’s construction and preparation for its first and final voyage was ending, 85,000 miles of open wire in the United States was loaded.  Cable, too, was loaded to the total of over 170,000 miles.  As we have spoken in Song elsewhere about Toll Entrance Cables, we’ll refresh the fact the majority of these loaded cables were inter-office trunk types.  At the time, over 125,000 loading coils were then in use.

Use of loading coils in conjunction with the birth of the telephone repeater, made further generous loading of open wire possible.  By the 1924 Presidential Election, 200,000 miles of open wire and 135,000 miles of cable were loaded.  Added up, over 777,000 loading coils were in use on the Bell System (according to their records) of which half a million coils were on inter-office trunk lines.  1,240,000 Western Electric loading coils were implemented around the various operating companies of the Bell System and its Long Lines Division.  This does NOT include the various thousands of Independent companies (as for example in Iowa, where companies numbered over 800 Independents!) using loading on their smaller systems where exchange and short distance toll were efficiently applied and activated.  Sounded like a good cause should go on . . . forever . . . right?

So, why did loading remain on cable and removed from open wire beginning in 1926?  Let’s look at the basics of cables vs. aerial wire.  If you go to the section on Aerial Toll Lead Obituaries, you’ll find a cross-section of the original A&B Cable used from Chicago to Omaha replacing the original open wire in 1936.  In your neighborhood, spot an aerial cable and if you cut it open, clearly exposed will be different insulated-colored conductors.  Typically, such cables contain either: 19-, 22-, 24-, or 26-guage conductors.  Obviously, these are quite small gauges.  As they become more distant from the C. O. or RST or RT (Remote Subscriber Terminal) their size diameter increases.  


However, with toll cables, where some distance passes between town to town, two pairs with identical electrical characteristics are transposed (twisted pairs) forming a four-wire group, or “quad.”  They are directional; meaning one pair transmits one direction “East” while the other “West.”  You’ll find it referred to as the “four-wire circuit” and carrier is most often utilized on these pairs. 

Capacitors are created when an insulator separates to conductive materials and electricity is applied.  Aerial copper open wire pairs are no different.  The two wires (and on large toll leads) the pairs above, below and beside them, perform this effect, as conductors are separated by an insulator–air.  So if the lead is a long one, capacitance increases.  Great insulators lessen capacitance and poor ones result in greater degrees of capacitance.

Cable circuits suffer far greater attenuation, or interference losses, than open wire for a multitude of practical and important reasons.  When you combine multiple pairs so densely packed in one sheath, higher attenuation results.  Now, if we introduce carrier, with its higher frequencies, then losses skyrocket.  But first things first . . .

You ask, “But wasn’t carrier first developed for open wire?”  Yes, it was.  Development of open wire carrier led to the use of impedance-matched carrier systems for cable.  However, loading coils tended to block higher frequencies, as were used in open wire carrier.  Also, higher speed telemetry circuits experienced problems with loaded circuits.  When a long-haul open wire circuit was loaded, even under perfect conditions, the transmission velocity was substantially slowed.  One major issue with these open wire long lines were the echo effects.  While echo cancelers were applied later to toll cable, the advance of carrier on toll carrier essentially rendered the loading coil unnecessary on lengthy routes.  Suburban and exchange carrier yet use them, but their use is decreasing.

Here’s how Conductor Size, Resistance, Frequency Attenuation and Capacitance impacted cable design:

Loaded Cable 19 Guage Losses Chart

Clearly, cables were shown to have higher losses than open wire lines.  While attempting to reduce distributed resistance by increasing the size of the cross-section of the metallic pair, might work (and does), now you have a very weighty cable and fewer pairs contained within the same sized sheath.  This impractical improvement has its benefits: distributed capacitance is cut by increasing space between them and in turn decreases capacitance along the whole run of the cable, too.  However, instead of a 300-pair cable along an expensive ROW/easement, you’ve got a far less efficient by-way for telecom.  Remember: in telecom traffic efficiency is number one priority.

For example, if loading coils had never been placed in the early years within the outside plant investment of long distance communications companies and larger diameter conductors were to take their place, it is estimated that the plant investment by 1930 would have been greater than one third of a billion dollars!  Think of the open wire facilities!  Using 16-gauge conductors, for example, would make crossarm mechanical stability necessary.  This would require heavier pins and arms.  Lots of costs pile up there.

What is the alternative to this “fewer pairs are better” arrangement?  Loading.  By increasing distributed inductance, attenuation is cut significantly.  What loading does is convert a cable with certain negative characteristics to one with improved electrical characteristics.  The word which is most important is “constant.”  Loading in voice frequency cable pairs allows the entire per-unit length to be static; predictable in quality throughout.  Between the various voice frequencies, there can be a maximum transfer of power, because loading is undertaken.


Now, open wire lines . . . don’t have the significant losses, or attenuation which cram-packed copper cable pairs experience.  Yes, there are losses, but are quite different and vary over time.  We talked about “constant” electrical characteristics of multi-pair cable with loading.  But, open wire is not protected within a sheath and therein lies the problem.  Exposed to the open air, these typically uninsulated conductors suffer the vagaries of climate: rain, sleet, snow (wet snow conducts; dry snow acts as an insulator), wind (which pitches the separation of pairs from one another as well as deflecting them  at points of contact with insulators), pairs hung above and beneath other circuits as well as electric power A.C. potential induction and contact, simply do not allow open wire to be loaded.

Here’s another peril: loading coils are impractical to increase open wire’s inductance as an efficiency method because we know how wet insulators impact attenuation–especially at higher frequencies–and no insulator is completely an “insulator”.  All have leakage losses of one form or another.  This bouncing “imbalance” between electrical characteristics versus cable pairs protected within a sheath, make loading really pretty impractical.

Another feature restricting their use on open wire was the reduction in line distortion effects.  To explain, loading open wire revealed the need for far better insulators at each pole attachment along with new procedures where separation between physical pairs was crucial.  A loaded line worked at higher voltage than non-loaded.  Because all insulators “leak,” there is a propensity for all insulators to shed a little voltage over their sides down to the pin and arm and to the pole when it rains.  Other contaminates also worked vengeance on open wire, such as dust, salt spray from the oceans and industrial pollution.  Steam trains were major perpetrators of the latter with their exhaust of black smoke billowing from their engine stacks and depositing upon insulators, un-insulated tie wires and cross arm pins. 

When the U. S. Transcontinental (original) link was being constructed in the early teens of the 20th Century, loading open wire furnished enough “umph!” to make transmission efficiency capable from New York to Denver.   However, loading open wire was substantially different than the existing cable lines at the time.


Open wire was loaded, but distances between loading coils was far closer making these aerial pairs very heavily loaded.  Also, as in this chapter heading photograph, the load coil cases were large, so they could accommodate very large coils–much bigger than cable’s similarly installed units. 

Open wire was much more susceptible to lightning attack than cable, thus they had a “breakdown” test strength at the early part of the last century at 8-kV.  Each coil was protected with lightning arresters on each side pair.

Also open wire, which in early use, was considered far more efficient at voice transmission than cable.  That suggested to the experts at that early date, the need for open wire loading coils to have smaller losses than underground cable coils was required.

Interestingly enough, however, transmission delay effects are quite remarkable.  Over cable, voice waves are slower than open wire leads.  Long distance toll cable will transmit a voice at 30,000 kilometers/second compared to open wire velocities of 300,000 kilometers/second.  If they are loaded, the speed decreases markedly.

Technically, we speak of “series resistance” per loop mile in cables, for example.  This resistance is based upon the cross section of the open wire or cable diameter.  Resistance lessens when you increase the area of the wire.  Unlike in power, where EHV (Extra High Voltage) transmission lines possess dual, tri- or quad- conductors, telephone pairs are treated somewhat differently.  So, can’t you decrease the resistance of the telephone circuit by having two wires (combining their cross sectional size) together?  Unfortunately . . . not . . . as the same flowing current is in both wires whereby it proceeds out one wire and reverses on the other.  That’s the reason per loop mile, the resistance is twice that of a single conductor.

Carrier.  Now you’ve further opened another pot of brew by introducing higher frequencies.  Even with cable pairs, circuits conveying signals above 15,000 cycles are not loaded.  Special circuit designs require research when a particular pair is selected for service to be “unloaded”.  A Local Loop Engineer inspects the MPLRs (Mechanized Pole Line Records) or similar record documentation in determining what pairs need to be eliminated from load coils along the proposed route and turning over a completed work order to have splicers free those pairs from load coils.

Some of you might have heard the expression “H-88” for loading.  Let’s briefly touch on this figure of speech.  Loading cable pairs at a thousand cycles creates different electrical effects depending upon conductor size.  I can remember designing projects in keeping with the H-88 loading  preference as facilities were specified served by buried multi-pair copper cable.  Just to give you a glance at other types of loading:

  • H-31
  • H-44
  • H-135
  • H-172
  • B-88
  • M-88
  • B-135, and of course,
  • H-88

Okay, okay . . . yeah . . . what does the “H,” “B,” and “M” prefix mean?  When I worked with Southwestern Bell, this question came up some years ago.  Let’s crack open the mystery.  A great outside plant transmission engineer I worked with gifted me an old chart.  I’ll share it with you, but I’ll make a few points first.

First, when open wire and later, cables were first introduced with loading attended to them, there was a profusion of loading types.  When I worked with cable design, H-88 was our preferred loading technique, as most of the facilities on copper paired cable were voice frequency.  Our major gauges in suburban Dallas, Ft. Worth, Wichita Falls, Houston areas were 24- and 26-gauge.  As open wire was in its death throws, and what open wire remained were simple one pair pristine bracket leads with no loading what-so-ever out in the boondocks, the vast majority of these other loading styles simply lapsed from non-use.  Hence, H-88 became the “preferred” loading style for most telecom companies, Independent, GTE or Bell. 

So here’s the lowdown: The letter stands for per/foot spacing; the number designator reveals the Millihenry (expected) inductance value.

  • A=700 feet
  • B=3,000 feet
  • C=929 feet
  • D=4,500 feet
  • E=5,575 feet
  • F=2,287 feet
  • H=6,000 feet
  • J=640 feet
  • X=680 feet 
  • Y=2,130 feet 

So . . . what’s the meaning of the “88”?  These numbers are inductance values: hence,

  • 18=18 Millihenrys
  • 22=22 Mh
  • 25=25 Mh
  • 31=31 Mh
  • 44=43 Mh*
  • 50=50 Mh
  • 63=63 Mh
  • 66=66 Mh
  • 88=88 Mh
  • 106=107 Mh*
  • 172=170 Mh*
  • 174=171* Mh

The * designates a margin of application, not erroneous figure.

The larger the coil, usually the longer spacing.  Some of these configurations shown above were used with open wire and are “antique” to say the least.  Reduced to the most useful and practical designator was H-88.  We used it because it possessed lower bandwidth (for voice frequency use), lower propagation velocity, lower loss and higher impedance.

For example:  With “LC” for load coil designation, a simplified one line cable design would look like this:

[C. O.] 0′<————–6,000′—————-[LC]——3,000′——[LC]——3,000′—–[LC]—>end

Because gauges in toll cables tend to be larger, such as 19- and sometimes 16-guage conductors, only the H-31, H-44, H-88, H-172 and B-88 are used.  What this mysterious nomenclature reveals is that loading is not the end-all solution to attenuation and that there is a practical limit to applying loading to cable.  Because as each coil is periodically inserted into the cable length, the series resistance and inductance is increased along the conductors.  Once you have increased the resistance to such a degree with excessive loading, then you’ve exceeded the whole purpose of these devices.  My understanding when designing cable loading was that 18,000 ft. was the most extensive allowable length.  This was because transmission gain maximums exceeded efficient operation of the cable pairs.

How were loading coils spaced?  This depended upon the gauge of the cable and the length from one end to the other.  If we used H-88 loading, for example, on exchange (near a suburban C.O.) and it was 24-gauge, loading coil spacing from the C. O. to the first coil would be 6,000 feet.  Cutoff frequency would be 3,700 cps.  Attenuation would expected to be only be 1.13 decibels per mile.  For 22-gauge, 6,000 feet spacing would remain the same, however, cutoff frequency would be less: 3,500 cps and .79 decibels per mile losses incurred.  Telephone people refer to the margin of installation as “hand-grenade accuracy.”  Meaning that a buffer of installation spacing should have been (in the early years) kept below 2% deviation.  Later, loading coils became much more efficient and 5% was allowed.  Open wire load points were very crucially sited; cable much less so, and had greater margin.

Loaded Cable 19 Guage Losses Chart

One further note.  In the early days of cable, from its introduction to around the 1940s, cables were not necessarily, as today, packed with copper pairs of the same gauge, i.e. where a ANTW-200 (buried) or BKTS-200 (strand-supporting aerial) cable were all 26-gauge pairs.  If you note the A&B Cable example in the “Obituaries of Open Wire” within this website, the cross-section reveals several different gauges.  This is no longer done and fell out of favor by the 1940s.   I worked on only a few cables which reflected this characteristic.

And . . . here’s the most unappreciated fact of all, regarding the loading of cable pairs vs. open wire.  Besides eliminating the “cut-off effect” (in the old days it was referred to as “Lumpiness” of coil loading in open wire) and where “unloading” allowed higher transmission velocity on aerial wire, greater long haul stability in with open wire was achieved.  This resulted because, in general, open wire had lower characteristic impedance and further freedom from speech distortion.  One attendant issue with loading was when you “load” or re-generate strength of a pair’s signal, you also re-generate the static–interference noise as well.  This is where repeaters, for digital systems, were introduced with considerable promise in the years to follow.  Echo effects and velocity distortion were troublesome in the early years, too.  With improvements in the newer materials used for loading coils, such as Permalloy, introduced by Bell Laboratories in the mid-1920s, the full potential of extending telecom across the continent and lowering costs was fully realized–for cables

Another point: in the early development of loading systems, engineers debated, “At what range of frequency should be transmitted?”  This fundamental question was considered and a “standard” cutoff frequency of 2.3 kc was adopted.

Internal Appearance of a Loading Coil

Let’s look at how these little buggers are constructed.  Our lesson today will also introduce some other features which make them quite useful devices for signal quality as well as improving efficiency of signal quantity (as in allowing as many cable pairs possible to be practically packed within a limited circumference insulated sheath).

Loading Coils Cross Section

For an efficient loading coil properties, the less winding resistance  the better.  A loading coil must have a coil with very low loss and a perfect equilibrium must be created by inductance.  To clarify this, consider a loading coil divided in two.  Since one wound energized wire is the “speaking circuit”, opposing it on the other side of the wound donut is a similarly wound circuit.  Both have an equal number (ratio) of turns.  Around the core traveled a magnetic field created by two windings.  When current reversal occurred, the circuit experienced “inertia” or inductance to the circuit.  The reason for the toroid shape makes magnetizing of the coil easily with as few losses as possible.  

By the way, each “pot” was not only installed with coils, but surrounded and bathed in insulating mineral oil, just as other electric power equipment saw similar application. These were hermetically sealed and no moisture was allowed to enter.  There was a gasket around the top case and several bolts held the cover on the larger units.  Smaller pole-mounted units were encased in lead with no option to open or inspect the interior coils.

Each “pot” contained a spindle upon which were piled these many individual coils.  Occasionally, loading coils were placed in cable splices.  I might also add, long spans of C-Rural Wire had installations of loading.  Let me dig up one from the collection we’ll photograph it here for you.  C-Rural Wire largely replaced open wire bracket leads.

C-Rural Wire Loading


This is a front view (rubber cover removed) of a Western Electric  model 178A1 Coil Case Unit.  These were very common in the 1960s when C-Wire began to take the place of open wire bracket leads.   Note the strap for fastening the unit to a pole with two top and bottom lag screws.  The oval holes on each side of the disk are for the C-Wire to be pulled through from the rear, crossed over and then spliced opposite the wire direction entrance to avoid wire strain.  Binding posts secured the C-wire pairs (tip and ring) when the wire stripped back and tightened.

While we’re on the topic of loading, it could be accomplished on the most minimal of circuits, not just multi-pair cables.  Since C-Wire (which is a plastic jacketed, very high strength drop-type paired conductor, similar to regular drop wire, but tough enough for long spans) is used to replace many open wire bracket leads, we’ll discuss how it was loaded.  GTE, the Independents and transportation companies all had a similar type “wire” and applied it to similar situations where open wire was removed and replaced by it.  If the C-Rural Wire followed a particularly long route, the application of a single pair loading coil was specified by an outside plant engineer.  Bell used the Western Electric 178A1 type loading coil.  It was extensively applied to C-Rural Wire, as I believe Reliable had something very similar to it for the Independent companies’ use.


Side view of same loading coil for C-Wire.  The diameter of the unit was about three and one fourth inches.  The the whole device top to bottom was five and one half inches.  Cover was made of flexible neoprene rubber and was just squeezed on the solid core.  No screws held it in place.

The little hand-held sized, resin-filled, loading coil for one pair was took on the appearance of a large biscuit.  It was about six inches in diameter and had a rubber cover, to be removed to access connections inside.  A mounting bracket formed the rear of the device support.  Two mounting screws, above and below, the device single metal bracket would allow it to be applied to a pole side, directly below the drive hook where the C-Rural Wire was dead-ended on “Pre-Formed C-Wire ends.”  The loops from both of the incoming and outgoing pair were cut and then threaded through the entrance holes for the wire.  This allowed for “drip loops” on each side of the coil entrances.

Impressed upon the rubber cover was specifying nomenclature for ready identification and the proper size selected for specific C-Wire placement. 


This is a rear view of same unit.  Note the whole unit was resin filled and had no access to the plastic encased metallic core and conductors.

What made this little load coil unique was “built-in” or integral protection to meet any surge currents from lightning or power contacts of the C-Rural Wire spans.  Two protector units by-passed the errant currents around the coil.  They were not ground-wire bonded to pole.

These were seen quite often in the early days, but it takes some searching to locate C-Rural Wire with loading today.  It’s out there where you might have seen former bracket lead open wire.  If you spot it, the miniature black Oreo-shaped loading coils quickly appear to the eye where particularly long span services cross to farmsteads.  I might also add: open wire lines often supported C-Rural Wire on crossarms.  It was quite common to see the C-Wire Support or “D-Wire Bracket” with pre-formed spirals, formed over the crossarm and the C-Rural Wire suspended directly under the ten-pin crossarm–typically between the 3rd and 4th or opposing, the 7th and 8th pin if two or more C-Wires are used.  No bolts, lags or preformed tie materials were used, simply the “C” shaped high strength steel unit.

C-Wire was a quick and dirty solution to avoiding placing another arm or bracket lead with minimal clearance to ground or to another arm beneath the top arm.

Squirrels never touched open wire in my experience.  It was greatly different when C-Wire was installed to replace it.  In Texas, we found squirrels LOVED chewing on it!  More outages were caused by this problem than open wire ever presented us with!  The little incisors of these rodents found C-Wire insulated jackets a choice selection when their ever-growing teeth needed to be worn down.  Another reason why open wire was superior in many cases to this alternative.  A little chewing and . . . pop! . . . you had a shorted pair.

What About Loading Telegraph Circuits?

What a great question!  Let’s go back to when the telephone and telegraph industry was maturing in the 1920s.  Telephone companies, as well as railroads who leased much of their open wire to telegraph companies such as Overland, Postal and Western Union, the virtues of compositing were clearly identified.  Many of these circuits in the early days were loaded and by simultaneous transmission of both telegraph and telephone on the same wires made much sense economically.  However, within a few years of implementing this process, a unique electrical effect was identified: “flutter.”

Flutter was produced by the cores of the loading coils applied to the circuits.  What happened in the course of combining telegraph and telephone signals was a current  “modulation” effect.  This was first encountered around 1921 and the race was on to eliminate this problem.  Since the problem wasn’t originating with the physical transmission line, but in the coil itself, clearly the design of the coil had its faults.

When you examine a loading coil, it has a “core” made of iron.  When experiments were conducted with high and low permeability iron, the “low” type demonstrated better operational efficiency.  This, in turn, allowed loading coils of lower permeability to be installed on major toll circuits where the conductor gauges were considerably larger.  The experience of using low permeability cores was a major improvement in cable and eliminated from open wire use.  A few years later, by the mid-1920s, air-gap loading coils were adopted as a further advancement in their design and “flutter” was substantially reduced with superimposed telegraph and telephone systems on nearly all long distance telephone routes. 

Powdered iron was used in the early loading coils and with long repeatered lines, worked advantageously.  This electrolytic-deposited iron, ground to a talcum powder density was insulated by an industrial process before compression into its characteristic ring shape.  This solid shape was the result of high pressure compression, but having little tensile strength.  Because solid iron cores were not used, these powdered cores possessed microscopic small air holes throughout.  While not mechanically strong, this characteristic allowed them to be very magnetically stable, thus eliminating magnetic leakage effects.  Being very stable, the inductance values were kept constant, even when external inductive effects of nearby A.C. power circuits, thunderstorm effects and minor short circuits, passed closely to the circuits. 


This photo, from 1903, illustrates the induction problem to nearby electric power and telecom/signal circuits.

It was found later, these stable cores could withstand a considerable amount of D. C. current temporarily.

% Change in LC Resistance vs.Current

The actual production of these small core assemblies was conducted very uniquely.  The iron particles were separated by cathode plates suspended in tanks, the purest iron removed, smashed into fine particles an inch square and ground up.  Each iron particle received an oxide coating and some shellac.  After one hundred tons of pressure, these rings emerged from pressing where 35,000,000,000 particles were contained in only seven (7!) small rings!  This greatly reduced magnetic instability and impedance was very uniform through this compaction.

The rings were then assembled one upon the other meeting the dimensions relative to their electrical application.  Typically, a paper covering of kraft or similar material was applied for protection between coil conductors.  In a large assembly, tightly confined within, were hundreds of these units, all connected individually to their pairs within a copper cable connection.  Needless to say, these very heavy units took up much space and were extremely heavy.  You may see them on poles or H-fixtures, or if you are lucky enough to visit a vault or manhole, deep on the floor of the hole.

Open Wire is Freed From Loading

Concern over circuit loading posed a problematic situation. M. I. Pupin, a recognized Professor of engineering at Columbia University, in early 1900s, invented an early style of coil loading.  What he, and Dr. G. A. Campbell of AT&T’s research arm accomplished, was if inductance coils were placed uniformly along a lead, these “lumped inductance” would render electrical qualities along its length largely uniform.  Here’s where the length of electrical waves and loading coil placement design coincided. 

One of the big problems with the coils was efficiency.   By 1904, the early cores were designed and built with iron cores made of spun iron wire.  A single core was wound with about ten miles of conductor it for a single core!  Each wire was lacquer coated for insulation.  This prevented eddy current losses.  A copper winding was then run on each half of the doughnut with leads extending out to connect with the EAST – WEST pairs.

Others, such as Thomas Shaw, further perfected this design technique around 1900.  When the issue of phantom loading came up and fear that loading might make its use challenging, first installations between Boston and Neponset, on multi-pair copper cable in 1910, presented no problems.

  Permalloy LC vs. Wound Iron Core Effic. Improvement Chart Iron wound vs. powdered  LC

Between 1904 and 1916,many new advances in constructing more efficient coils and cores overcame previous less efficient designs.  In 1916, the use of fine powdered iron compressed into rings and then each inserted above each other was the conventional process of manufacture.  Then came World War I.  Interestingly enough, the war in Europe had blockaded the diamonds imported for use in the drilling dies.  These dies allowed fine wire to be drawn drawn through the iron cores.  The problem became so serious that future production of the previously manufactured type appeared impossible. 

By 1926, designers of toll open wire were backing down on further installations to load them.  Bell, and other major carriers, eliminated it from new aerial wire facilities being built.  In those days, the major open wire toll conductor gauges were primarily 165 mil and 104 mil high strength copper.  The repeater was making its inaugural appearance by 1915 and open wire carrier was being slowly introduced for the first time.  By 1922, some advanced work on carrier was underway.  With the higher frequencies of carrier systems, loading stood in the way of effective carrier transmission over open wire.  Another roadblock to applying it further to toll open wire.

Loading Systms-Small Gauge Repeatered Toll CablesEarly information on phantom circuits exposed to repeaters and their effects, 1927.

Repeaters’ development offered the first electronics dramatically exceeding lengths of conventionally constructed open wire toll wire.  Repeaters were also introduced into cable circuits with amazing success.  With their first cable installations, cable lengths were pretty short, contained large gauge conductor pairs and grounded telegraph was superimposed upon the telephone pairs.  Loading coils were then ample in affecting efficient use of these multiple use lines.  And, let me suggest, even with the advent of the first electronic repeaters, there was no fundamental changes to engineering existing loading coils, except to be more precise as to intermediate physical locations when installed on these early short-haul cable leads.

Effic. Improvement Chart Iron wound vs. powdered  LC Repeater Transmitting Gain

Distortion effects, typically caused by grounded telegraph circuits, were slowly eliminated when low permeability type cores were used.  Further advances in air-gap units, also made their installation throughout the United States and Canada opportune.

One other factor influenced open wire’s freedom from loading: series inductance.  When open wire lines were constructed, depending upon their transmission purpose, early pair spacing was uniform nearly everywhere based on the 1885 design.  Later, with the advancement of carrier, efficiencies were found where varying this distance was necessary for proper operation.  Original open wire lines, 12-inch between pairs and 16-inches between pole pairs, was a given.  Later, requirements for higher efficiencies dictated the use of 8- and 6-inch spacing and elimination of the pole pairs and phantom pairs.  Cable was quite different.  Crammed into the sheath were pairs which typically were twice the insulation thickness.  So per loop mile, the inductance variation required engineering for operation with and without carrier.

Permalloy Loading Coils

The efficiency created by the use of loading coils was well established, but as more cable was introduced into long distance outside plant in the early 1900s, applied also to inter-office trunking from open wire toll, pioneering work began to perfect the loading coil further.

On the Transcontinental U. S. open wire lead, the loading coil cases for these strands across the Western United States, were substantial in weight and size.  Pacific Telephone & Telegraph regularly installed pots containing three loading coils: two for side circuits and one for the phantom of all four wires.

But by 1926, the size of the coil was substantially reduced, and economies were gained by smaller case sizes where attached to cable poles, manholes and H-fixtures.  This came about because of the innovative “Permalloy” introduction by Bell Laboratories.

Essentially, “Permalloy,” is a patented AT&T trade name for a specialty bi-metallic material combining 80% nickel and 20% iron compounds.  A guy at Bell Labs by the name of G. W. Elmen, discovered it. He decided to try a practical application and found use when loading of the transoceanic telegraph lines.  Here the permeability factor–which in Permalloy had highly developed magnetic qualities–was concentrically wound around the telegraph wire in the submarine cable.  When installed between Atlantic coasts and heated up, what telegraphers immediately discovered as signaling speed rose dramatically.  More telegraphic traffic raced over such a loaded condition, making for many more communications possible than the previously laid conventional submarine cables.

Engineers deduced by using Permalloy in conventional terrestrial cable routes, similarly a parallel advantage might also result.  The promise of the loading material was obvious.  However, clearly, the submarine cable process had many changes in store before it could be applied to aerial cable crossing land.  What caused such positive notice was the material’s low losses–particularly the form of “hysteresis” loss.  What hysteresis means is when magnetization results within the core, there is a lag or slowing, relative to the magnetizing force.

Soon, by the mid-1920s, the practical device whereby the Permalloy was used, was achieved and industrially produced for application to aerial cable lines of significant length.  And as many open wire toll circuits were being converted to cable, its application to the loading of open wire simply diminished . . . and faded.

“Frogging” Circuits

Let’s leap frog to the relatively humorous–but practical topic of creating electrical equivalency in open wire or cable wires by linemen, installers and splicers.  I’ve heard this term from old-time open wire linemen, installers, splicers and railroad signalmen often.  Having asked what this odd expression meant, I can assure you, as old as the hills it indeed is, it survives today as a common day phrase!  What a splicer is doing with a cable pair by “frogging” can be, for example, at the end repeater sections, when the distance between repeaters exceeds a desired half-section.  Perhaps, in this situation, 19-guage circuits might be brought in over 16-gauge circuits in this last repeater section.  In open wire, a 12-guage might be “frogged” with 8-gauge in cases where a long repeater section may be encountered and the use of 12-gauge won’t provide the equivalent necessary.

Below is an interesting example of “Loading” in early radio.  This page is from a 1926 Ohio Brass Company catalogue.


Travelling Waves on Transmission Lines: Inductive Loading

By Tom Hagen


This article is a continuation of the second article on transmission lines, specifically covering issues affecting transmission of frequencies on open wire telephone lines.

Information I wish to acquaint you with through this article.:

More results from Traveling Wave Transmission Modelling:

  • Traveling waves on transmission lines
  • Phase distortion problems on transmission lines
  • Lossless lines; distortionless lines

Some practical examples:

  • Michael Pupin and others
  • Loading coils on telephone lines
  • Distributed inductance to improve telegraph cable speed


In previous articles, I’ve written about how the development of long undersea telegraph lines drove the need to deal with the effects which degrade signal propagation on long lines (See chapter on “Telegraph on Open Wire Lines”.  The development of electromagnetic field theory by the “Maxwellians” was instrumental in continuing this work in the telephone technological field.  Audio alternating frequencies are passed by telephone lines to represent speech in the 300-3,000 Hz frequency range.  Electromagnetic field theory illustrates these A.C. signals are actually electromagnetic waves travelling in the space between the wires, and are not represented as voltages and currents flowing in the wires.  

One result of this new theory was higher frequencies travel down the phone lines faster than at lower frequencies.  This effect is called “phase delay” or “phase distortion.”  On longer lines, this causes distortion of the voice signal because the time relationships of the original signal are lost, thus causing phase distortion.  Remedies in the form of inductive loading coils are applied to address this condition.  Loading coils are placed at regular intervals on phone lines to reduce both phase distortion and attenuation of higher frequencies in the voice range.  Another desirable effect of loading coils is reducing the attenuation, or reduction, of the signal strength over the length of the long open wire pair. The theory behind this development was first elucidated by Oliver Heaviside.  Michael Pupin patented the loading coil.

Travelling Waves on Transmission Lines

A single frequency sine wave signal, say of 1kHz frequency, travels down an open wire pair transmission line and can be represented as in the diagram below:

Michael Pupin

The magnetic field from the current flowing in the transmission line wires is at all points at right angles to the electric field lines from the opposite charges on the line pair.  Under these conditions, the math explaining the field conditions on the transmission line show energy is transmitted down the line in the form of traveling waves.  These waves travel at near the speed of light, or around 186,000 miles per second.  Note that free electrons in the wires simply provide the conditions for the traveling wave to exist.  The electrons themselves just jitter back and forth on the line as the waves pass by.  I would like to compare this to striking a line of billiard balls.  The impulse shoots through the line of balls, but the balls themselves barely move due to the impulse.

Without going into the fairly involved math (differential equations, real, complex, and imaginary numbers and so on), I’ll just present the results.  There are plenty of sources online and within books on transmission line theory, if you wish to delve further into transmission line mathematics.

But first, we need to examine a circuit model of a transmission line.  The simplest transmission line is a pair of parallel wires.  Currents flow in opposite directions on each wire and there is a potential difference, or voltage, between these conductors.

The circuit below represents a tiny chunk of this transmission line.  There is a series inductance in each wire making a magnetic field around the wire (magnetic fields form around current-carrying wires), capacitance between the wires (electric fields form between the wires when they have opposing voltage potentials applied to them).  Then, there is series resistance of the wires themselves, represented by the series resistor in the diagram.  And, finally there is leakage current between the wires represented by the resistor connected across the transmission line pair.  The constants are represented by the symbols L, C, R and G, respectively.  L is inductance, a unit of Henry, C is capacitance in Farad, R is resistance in Ohm, and G is conductance in Siemen.  The “dx” element represents infinitesimal distance, as the length of the transmission line element is very, very small.  The entire length of the line is composed of an infinite number of these infinitely tiny chunks strung together in series.  The four parameters are evenly distributed along the length of the line and are specified as “Henry/meter,” “Ohm/meter,” “Farad/meter” and “Siemen/meter.”  The behavior of traveling waves on this type line is dictated by the value of these distributed parameters.  


One of the interesting results of this theory is if the line is “lossy,” (or has values of R & G greater than zero), then higher frequencies travel faster on the line than lower frequencies. This causes phase distortion.  Another effect of the lossy line is higher frequencies are attenuated more than the lower frequencies.  Recall: attenuation is signal loss that occurs along the length of the line.

Inductive Loading on Transmission Lines 

The mathematics proving these assertions I’ve previously mentioned, was developed from the simple distributed parameter model and is not trivial (See the Wikipedia article on Transmission Lines to flavor the mathematical details).  Oliver Heaviside and the Maxwellians, in addition to developing the transmission line mathematics, arrived at a solution for the attenuation and phase distortion problems by proposing what is called the “Heaviside Condition for a ‘lossless line'”.

The equations show that if 

RLGCGreater Than


the velocity and attenuation are no longer functions of frequency, which definitely is the case for a lossy line.

Typically, in the real world, 

RGLC second

for a lossy line, so that if L is somehow increased along a telephone line, the R/L term is reduced in value and can be made to be equal to the G/C term.

Another benefit that drops out of this technique is attenuation is made to be constant across the frequency range of the line.

Now, as we have the concept of line attenuation and phase distortion clarified, let’s talk about remedying this situation.  Michael Pupin of Columbia University patented evenly distributed loading coils and made a fortune out of it.  He published a significant paper in 1900 detailing the use of loading coils and also patented these coils in 1899.

Michael PupinMichael Pupin (1858-1935)  

He became a wealthy man when AT&T bought out his loading coil patent in order to avoid patent interference with Bell’s own loading coil development work.

How are inductive loading coils used on telephone lines?  In our figure below, the coils are made up of toroidal ferromagnetic material and both wires of the pair are wrapped around the cores to provide series inductance at regular intervals.  Coil spacing on long phone lines may be on the order of 6,000 feet.

Series Schematic of Loading coils color

See the many fine references and pictures of real loading coils in other sections of the Song website.

A problem with using loading coils is they make the phone line behave like a low pass filter.  When the coil values and coil spacing are optimized to minimize distortion and attenuation in the voice frequency range (approximately 300-3,000 Hz), the frequencies above 3 kHz are sharply attenuated and the line cannot pass frequencies above 3kHz.  This would not normally present a problem, but when phone companies began providing digital service via digital subscriber line (DSL), the lines proved to be inadequate to pass the higher frequencies required.  DSL can use frequencies as high as 4 mHz, and loading coils must be removed from the lines in order for DSL to function.  

Another service, very high bit rate digital subscriber line (VDSL), requires up to 12 mHz bandwidth for a maximum of 52 Mbit/s data rate.  Note: the highest VDSL speeds can only be achieved out to about 1,000 feet from the phone line pedestal/terminal.  This limit is set by line loss which at the higher frequencies is caused mainly by leakage conductance and insulation dielectric loss between the two conductors of the phone line pair.

As an aside, undersea telegraph cables used continuous inductive loading to overcome the same distortion effects of telephone lines.  Without loading, cable speeds were limited to perhaps 400-500 letters per minute.

Cable cross section in color single conductor undersea

The inductive loading was done with a material called mu-metal, having very high magnetic permeability.  This property allows mu-metal to absorb and retain magnetic fields much more readily than air or other non-magnetic material surrounding the conductor which it is wound about.  With the mu-metal loading wire wound around the center conductor, the series inductance is significantly increased and the Heaviside condition of:


is met.  Since the mu-metal winding provides continuous inductive loading, the telegraph cable does not act as a low pass filter.  Inductive loading of telephone lines in this manner is expensive and normally not done.  Also: having a cutoff frequency above the highest voice frequencies was not a problem until the advent of phone line multiplexing, frequencies above 3kHz were not required on telephone circuits.

With inductive loading, speeds of 1,900 letters per minute became possible, or about four times the speed of unloaded cables.  Note that 1,900 letters per minute corresponds to about 2,000 bits per second.  By comparison, today’s undersea fiber optic cables transfer upwards of 15 trillion bits per second on a twelve-pair cable.

See also Tom Hagen’s contribution at: