286 Mr. W. H. Preece on some Physical Points 



detect the presence of extremely feeble currents from doubtful 

 sources of electricity. If currents from the supposed source 

 be rapidly sent through one wire of a double-wound coil, and 

 a telephone be fixed on the other running parallel to it, then 

 the telephone w r ould give evidence of their presence, which 

 would be indiscernible on any other instrument. 



It is admirably adapted also for testing leaky insulators 

 and supports. 



VI. Inferences and Results. 



The telephone explodes the notion that iron takes time to 

 magnetize and time to demagnetize. If time w T ere occupied 

 in magnetizing, notes would be changed or lost ; but they are 

 not altered. The notion of time is due to the action of in- 

 duction in coils producing reaction and extra currents. This 

 is proved by the insertion of an electro-magnet or of coils of 

 wire in a telephonic circuit. While it is possible to speak 

 through a cable 100 miles long laid out straight in the sea, 

 it is impossible to speak through 20 miles when coiled in a 

 tank. 



Its delicacy has detected the presence of currents in wires 

 contiguous to wires conveying currents, which have always 

 been suspected, but have been evident only on wires running 

 side by side for several miles (say two hundred) on poles or 

 in well insulated cables. In fact, the most delicate apparatus 

 has hitherto failed to detect the presence of these currents by 

 induction in short underground wires ; but the telephone re- 

 sponds to these currents when the wires run parallel for only 

 a few feet. Thus, between one floor and another floor, at the 

 General Post Office, it has been impossible to converse by 

 means of the telephone through a wire, owing to the presence 

 of these currents of induction from the innumerable working- 

 wires contiguous to it, and through some of the underground 

 pipes of the streets of London sounds are inaudible when the 

 wires are working. In fact, two small-sized gutta-percha 

 wires, one foot long, were lashed side by side by Mr. Marson ; 

 and when battery currents were sent through one, induction 

 currents were distinctly heard on a telephone fixed on the 

 other. Indeed this induction between wire and wire, has 

 proved the most serious obstacle to the practical introduction 

 of the instrument. But it is not altogether irremediable on 

 underground wires ; it can be surmounted in three ways : — 



1. By increasing the intensity of the transmitted currents 

 so as to overpower the currents of induction, and by reducing 

 the sensitiveness of the receiving apparatus so as to make the 

 instrument insensible to currents of induction though responsive 

 to telephonic currents. 



