Publication: Electrical World
New York, NY, United States
Overhead Circuits for Long-Distance Telephony.*
BY M. J. BANNEUX (CHIEF ENGINEER OF THE BELGIAN TELEGRAPHS).
For long-distance telephony a metallic circuit is an absolute necessity, as with a single wire and an earth return it is impossible to avoid the noises due — first, to earth currents; second, to induction from neighboring wires; third, to the '"picking up " of the currents from other circuits in the vicinity working also with earth returns. As an example of the last the author mentions an overhead wire in Brussels which made earth some fifty yards from a Morse instrument. The two wires ran at right angles to each other, so that there could be no appreciable induction between them, yet every Morse signal could be heard on the telephone line.
The best method of stopping cross-talk, due to leakage, is, the author considers, the English method of using good insulators and joining all their iron supports to an earth wire at each station; most of the leakage current then gets to the earth instead of into the other wires.
Induction from wire to wire is the other béte noir of telephonists, and to it many operators attribute cross-talk which is really due to leakage. It is true, however, that in foggy climates it is very difficult to determine which is the real offender.
The effect of one wire on another is represented approximately by the formula,
where m is the coefficient of mutual induction, l the length of the two wires running parallel, and d their distance apart. The general solution of the difficulty of induction, whatever may be the inducing causes, is to use two wires in a complete metallic loop instead of a single wire and an earth return, and to so arrange them that the sum of the inductions of neighboring telegraph or telephone wires shall be exactly the same on both the wires of the loop. This principle was patented by Bell in 1877, and its efficacy was proved by Hughes in 1879.†
It was applied by Brooks, of Philadelphia, to cables, by twisting the two wires of the loop together.
If a line of poles only carries one telegraph wire and a looped telephone wire it is very easy to obtain equal induction on both wires of the loop; all that is necessary is to run the telegraph wire parallel to its neighbors and equidistant from both of them. The telegraph wires, however, in practice are numerous, and vary in number from section to section, and the telephone wires cannot be arranged so as to be equidistant from each particular telegraph wire. In this case it is necessary to adopt a method similar to that employed for cables, namely, to twist the wires together in long helices. Prof. Hughes suggested a plan for doing this in 1879, which was carried out by Messrs. Moseley & Sons near Manchester. Since 1881 the English post-office have used the wiring shown in Fig. 1 for the trunk wires for joining two towns. It two trunks have to be run on the same line of poles they are arranged as in Fig. 2. Galvanized iron lines on this system are at work between Manchester and Liverpool, 30 miles; Newport and Cardiff, 13 miles; Glasgow and Greenook, 25 miles; Glasgow and Edinburgh, 36 miles; London and Brighton, 54 miles, etc., etc.
In June and July, 1883, the Belgian Government established communication between Antwerp and Brussels, which towns, by the way, were the first two on the continent which were joined by