5 



NATURE 



{Sept. 21, 1882 



upon lofty separate poles and away from all other wires, there 

 would be no difficulty whatever in speaking between th jse tw 

 place*. Conversation has been held in America over 410 miles ; 

 in Persia it has been effected between Tabreez and Tiflis, 390 

 miles apart ; in India, over a dis'ance of nearly 503 miles ; in 

 Australia, of 300 miles ; but in all these cases it was done either 

 at night or under exceptional circumstances, and in all cases the 

 wires were over-ground. Had they been underground or sub- 

 marine, the case would have been very different. Conversations 

 have been held between Dover and Calais, between Dartmouth 

 and Guernsey, and between Holyhead and Dublin, but I know 

 of no case where any persons have spoken through more than 

 100 miles of submerged .cable. The reason of this diminution 

 of speaking distance is due to the electrostatic capacity of the 

 telegraph line, which absorbs the minute quantity of electricity 

 that makes up the currents employed for telephonic purposes. 



In every submarine cable, before a signal can be made at the 

 receiving end, the whole cable must be charged up with 

 electricity, and if there be not sufficient electricity sent in to 

 effect this purpo e, practically no signal appears at the distant 

 end. With telephone currents on long cables the whole of the 

 electricity is, as it were, sw allowed up — that is, none appears at 

 the distant end, or, if it does appear, it is rolled up in one con- 

 tinuous wave, bereft of those rapid variations that reproduce 

 sonorous vibrations. The newspapers said that the sound of the 

 bombardment of Alexandria was heard at Malta ; but, in the 

 first place, the experiment was not tried, and, even if it had been 

 tried, it could not have succeeded. The use of underground 

 wires very seriously impedes telephonic extensions, and with our 

 present apparatus and present knowledge we cannot readily 

 speak over greater distances than 20 miles. 



Disturbances. — But there are other disturbing influences at 

 work of more serious import. 



When two or more telephone wires run side by side, what is 

 s lid on one can be overheard on all the others ; and when a 

 telephone wire extends alongside telegraph wires, every current 

 on the telegraph circuit is repeated in the telephone, leading to a 

 hissing, frying, bubbling sound that is not only very irritating, 

 but which on busy lines entirely drowns speech. When music 

 is transmitted on one wire, it can be heard equally well on all 

 wires running parallel and contiguous. This is due to induction 

 and to leakage. 



(a.) Induction. — Induction is a term employed to designate 

 the peculiar influence which electrified and magnetised bodies 

 exert upon conducting and magnetic masses in their neighbour- 

 hood. If two wires run side by side for some distance, every 

 current of electricity sent upon one wire will produce two 

 currents in the contiguous wire, the one at the commencement 

 and the other at the end of the primary current of electricity. 

 The greater the intensity, and the more sudden and abrupt the 

 commencement and the ending of the inducing current, the 

 gi eater effect it has on the induced wire. Those instruments, 

 consequently, which reverse their currents the most rapidly and 

 suddenly, produce the greatest disturbance. 1he_ powerful 

 alternative and intermittent currents used for certain electric 

 light systems are death to telephones : they cause an incessant 

 roar that renders speech an impossibility. There are some 

 apparatus in telegraphy that require very powerful currents to 

 work them, which are equally detrimental. Many attempts have 

 been made to cure this evil. 



1. The sensitiveness of the receiver has been reduced to lessen 

 the influence of the disturbing currents, and the strength of the 

 telephonic transmitting currents has been increased so as to over- 

 power the induced currents. 



2. The influence of one wire on the other has been screened 

 off by inserting metal coverings in connection with the earth 

 between them. 



3. The suddenness of the rise and fall of the inducing currents 

 has been modified by the insertion of condensers or electro- 

 magnets. 



4. Counterbalancing or neutralising effects have been set up 

 by counter-induction apparatus. 



But all these plans, and many others, have been proved either 

 only partially successful or wholly abortive ; the only effective 

 mode of curing the evil at present practically used is to employ 

 a complete metallic circuit so contrived that the two wires are in 

 very close proximity to each other, or that they twist round each 

 other, so as to maintain a mean average equality of distance 

 be ween themselves and the disturbing wires. When we have 

 the two wires of a circuit kept at the same mean distance from 



the disturbing causes, however near they may be, the influence 

 on each must be identically the same, and as the one is used for 

 going and the other for returning, the similar influences must be 

 opposite in direction, and they must therefore neuti alise each 

 other. This plan, which was originally devised for underground 

 wires by Mr. Brooks, of Philadelphia, was found to be 

 absolutely true in practice, and the Post Office, having laid down 

 many hundred of miles on this system with perfect success, 

 invariably constructs its circuits both underground or overground 

 in this way. It is, of course, more expensive than a single wire, 

 but the great gain— the absolute freedom from overhearing, the 

 privacy and the absence of crackling — is well worth the extra 

 cost. Wires in submarine cables are invai iably laid up with a 

 twist, so that no special contrivance is needed on such wires, and 

 in underground wires not laid up together as cables, they arc as a 

 rule, so close to each other that twisting is unnecessary ; but for 

 overground purposes tw i-ting is essential, and special arrange- 

 ments have to be carried out. Professor Hughes showed how 

 this was to be done, and Messrs. Moseley carried it out practically 

 in the neighbourhood of Manchester. The plan adopted by the 

 Post Office for two and for four wires is shown by the diagram. 

 It is simply and easily Carrie 1 out; and entails no practical 

 difficulty whatever. 



In the neighbourhood of Manchester there are over 400 miles 

 of overground double wire twisted on this plan, working 

 efficiently and thoroughly. I have spoken to a friend 76 miles 

 off, through wires that were erected on poles earring busily- 

 occpied telegraphic currents, without disturbance or difficulty. 



(n.) Leakage. — The double-wire system is only absolutely 

 effective so long as the insulation is good. The moment insula- 

 tion fails, connection with the earth is made, and then we have 

 disturbing causes due to currents flowing through the ground, 

 which are increased in proportion to the deterioration cf the 

 insulation. Hence, good insulation is essential to telephone 

 working. 



The discovery of the telephone has made us acquainted with 

 another phenomenon. It has enabled us to establish beyond 

 doubt the fact that currents of electricty actually traverse the 

 earth's crust. The theory that the earth acts as a great reservoir 

 for electricity may be placed in the physicist's waste- paper 

 basket, with phlogiston, the materiality of light, and other 

 hypotheses. Telephones have been fixed upon a wire passing 

 from the ground floor to the top floor of a large building, the 

 gas pipes being used as a return, and the Morse signals sent from 

 a telegraph office 250 yards away have been distinctly read ; in 

 fact, if the gas and water systems be used, it is impossible to 

 exclude telegraphic signals from the telephone circuit. There 

 are several cases on record of telephone circuits miles away 

 from any telegraph wires, but in a line with the earth terminals, 

 picking up telegraphic signals. When an electric light system 

 uses the earth, it is stoppage to all telephonic communication in 

 its neighbourhood. The whole telephonic communication of 

 Manchester was one day broken down from this cause, and in 

 the City of London the effect was at one time so strong as not 

 only to destroy telephonic communication, but to ring the bells. 

 A telephone circuit using the earth for return acts as a shunt to 

 the earth, picking up the currents that are passing, in proportion 

 to the relative resistances of the earth and the wire. The earth 

 offers resistance, and consequently obeys the law of Ohm ; 

 hence it is not only essential for a telephonic system that the earth 

 should not be used on any electric light system, but it is al-o 

 desirable that the earth should be eschewed for telephonic pur- 

 poses. Thus, the double-wire system adopted by the Post Office 

 and by the Societe' Generate des Telephones of Paris, not only 

 cures the ill effects of induction, but it materially diminishes the 

 disturbing influences of earth conduction. The four-wire system 

 of the Post Office effectually checks leakage from one wire 

 to the other — cross contact, as we call it in England — for each 

 wire of the same current is always on a different supporting arm. 

 A telephone circuit when in connection with the earth gives 

 distinct evidence of every visible flash of lightning, however far 

 off the thunderstorm may be. No difference in time has been 

 observed between seeing the flash and hearing the cra>h. 



It is said that, if a telephone be connected between the gas 

 and water systems of a house, distinct evidence of every flash 

 can be heard. There have been several cases of persons being 

 knocked down while experimenting during a thunderstorm, but 

 no personal injury has been sustained, although the apparatus 

 itself is frequently damaged. In England, at present, we have 

 not found the damage done sufficient to justify the employment 



