Publication: Journal of the Society of Telegraph Engineers
INDIAN AND AMERICAN TELEGRAPHS.
In March, 1873, Mr. W. E. Ayrton read a paper, entitled " On some Points in connection with the Indian Telegraphs," before the Society of Telegraph Engineers. This paper is published in No. 5, vol ii. of the Journal of the Society.
Mr. Ayrton first refers to the application of Mr. Schwendler's formula for reducing the apparent or observed resistance of either insulation or conductivity to its true value. Soon after the publication of Mr. Schwendler's formula I applied it in the measurement of insulation and conductivity of overland lines, but never found it to agree, or even approximate, to the result of a careful measurement, taken under circumstances where the insulation resistance was so large as not to affect the result, as measured in the ordinary manner; that is, to take the mean of the observed resistance with alternating poles of the battery.
Mr. Schwendler's formula is based upon the supposition that the insulation conductivities are uniformly distributed throughout the line, a condition of affairs that never exists in this country except in clear or cold weather, and when the escape of current by the insulators is so small as not to enter into the result.
In summer, long lines are often affected by local showers: it may be clear on some portions, and raining at others. During general rains, "north-east storms," humidity that affects insulators is very unevenly distributed — raining in some portions and only cloudy in others. The conductivity of insulation is never stationary under these circumstances, but as varying as the amount of rainfall at different intervals. I refer to the ordinary glass insulators of this country.
On very old wires, whose resistance is great through bad joints, conductivity is immensely improved by rain or dampness. So that, in my experience, it is idle to attempt the measurement of wire resistances or line conductivities, except by short sections or under favourable conditions of weather.
Mr. Ayrton says, "We have learned two very important facts," one of which is, "that the insulation of sections varies enormously under the same climatic influences." If this be true, how do they arrive at the real conductivity of the wire ? It also speaks badly for the insulators, provided we are to understand by the term "same climatic influence" that there is sensible leakage on any portion in clear weather.
A porous porcelain insulator conducts in clear weather; a glass insulator under same circumstances does not . By this is meant, when both are exposed to the sun's rays and not affected by moisture on the surface.
Mr. Ayrton enters into a calculation by which he is led to believe the consumption of battery material is increased, in his particular case, 60 per cent. by the introduction of the 1 per cent. of bad insulators. This calculation leads to erroneous results, inasmuch as it is based upon the supposition that there is no consumption when the circuit is open or the exterior resistance is infinite. Consumption of material is going on when the circuit is open, and this is particularly the case with the kind of battery he refers to as used in India, the Menotti.
In our country this battery without the intervening layer of sand or sawdust is most used. It is best adapted to our mode of working the closed circuit system. The size of the elements is sufficient to bring the resistance down to five and sometimes two Siemens' units per cell. If the battery is much worked this fact tends to keep the solutions separate. The more wires worked from the same battery, or the less the exterior resistance, the less the local action, and in this sense the more it is worked the greater the economy.
It answers admirably for working many lines from the same battery.
This battery is called the Callaud, and is essentially the same as used by the French at their central station in Paris.
Mr. Ayrton describes the manner of testing insulators at the factory and upon their arrival in Bombay, showing the great care bestowed upon the insulation