Of N&W White Porcelain Insulators & the Hygroscopicity of Glass

[NW Mailing List]

Probably no reader of the N&W List is unaware that the N&W (like many other railroads) marked its Train Dispatchers' telephone pair on pole line by the use of white porcelain insulators on the cross arms. 

For some time it has bothered me that older photographs do not show white insulators on pole line. I have come to a tentative conclusion as to why this is so, and would like to share a few points with the List, for the comment of others. 

The earliest insulators (used in the building of the first telegraph line, Baltimore-Washington, 1843-1844,) were porcelain bureau knobs (i.e. drawer-pull knobs.) Very quickly thereafter, baked clay insulators came into use on the nation's telegraph lines. In the 1860s, glass production technology had reached the level where insulators made from glass could be produced more cheaply and uniformly, and glass insulators were more resistant to breakage than those made from clay. 

No pole insulator, however, is perfect. Under any moisture condition other than perfect dryness, each "insulator" represents the potential for the "leakage" of small amounts of current to ground (through a wet insulator pin, cross arm and pole.) Using an average spacing of 32 poles per mile, across a 100 mile Division, a wire would be subject to 3,200 potential paths of leakage to ground. 

This potential for leakage of current was found to result from the "hygroscopic" properties of the surface of the glass. "Hygroscopicity" is the ability of a substance to attract and hold water molecules from the surrounding environment. 

Any reading of the literature of the 1870s-1890s will show that engineers struggled with the leakage problem, trying different glasses and different shapes for insulators, and by the late 1890s had achieved solutions satisfactory for the relatively low voltages used on telegraph lines (and early signal circuit lines, as well.) The old telegraph circuits operated on perhaps 60 volts DC, and early signal circuits on much less. (By comparison, 500,000 volt transmission lines can be seen in any part of the country today, and 740,000 volt transmission lines are being planned.  The higher the voltage, the greater the danger of leakage, which is called "flash over" in the electric utility business.)

During the 1890s, electrical transmission lines were constructed. The original electrical transmission voltage had been 100 volts (Edison, 1871,) but by the 1890s, when the transmission of several thousand volts was attempted, it became apparent that "leakage" of voltage to ground on glass insulators was a limiting factor. And thus it was that engineers returned to porcelain (originally called "flint glass") as a material for insulators. It was found that, by combining kaolin, feldspar and silica, applying a glaze, and firing the product at a high temperature, an insulator could be produced with hygroscopic properties far superior to those of glass. The disadvantage of porcelain was the high cost of production vis-a-vis glass. 

And here the railroads come into play... Shortly after the electric generating industry had developed a porcelain insulator with superior hygroscopic properties, the railroads began tinkering with the dispatching of trains by that new-fangled device, the telephone. The first experiments with this on the N&W, as far as I have been able to determine, occurred around 1910, or just shortly before. 

Early telephone circuits struggled with the problem of low amplitude (low voltage) signals, and thus the problem of leakage was critical. The higher telephone voltages allowed by the "telephone amplifier" was still in the future, and was not developed until Lee DeForest invented the vacuum tube and amplification became possible. (The first railroad "amplified" telephone equipment was marketed in 1917, I believe.)  THEREFORE, I conclude, when railroads began installing telephone Train Dispatching lines, obtaining insulators with the very best hygroscopic properties was imperative, and when the new telephone lines were installed, the newer, better porcelain insulators were utilized. This is my theory, and it explains why white porcelain insulators do not appear in early N&W photographs. 

Why WHITE porcelain insulators on the Train Wire pair? Perhaps to mark the Train Wire circuit for easy visual identification, so that anyone with a lineman's set could clip onto the pair and talk to the Train Dispatcher. By contrast, when railroads began to "hang power" on their pole lines, they tended to use brown porcelain insulators on the power circuits, not white, from my observations. The "hanging of power" on railroad pole line did not occur until railroads began to install AC track circuits and AC signaling in the 1911-1915 era, and generally the voltage was 440 volts AC. 

I offer these conjectures and ask for comment by those who may have seen photographic or documentary evidence pertaining to the installation/use of white porcelain insulators on N&W pole line. 

(  P.S. --  As a sweetener to those interested in signal problems...  Hygroscopicity was also a problem in the construction of signal relays.  Vitreous porcelain was also used for the top plate on the first glass cased signal relays, and it was soon found that an accumulation of soot, cinders, dust and mould on the top plate, by virtue of hygroscopicity, diminished the insulating properties of the porcelain and offered a "leakage path" for electrons from binding post to binding post.  To address this, the porcelain tops of many old relays were painted with a black substance called "asphaltum," which was supposed to reduce the hygroscopic electrical leakage.)

-- abram burnett 
"SW" Telegraph Office 

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