ELECTRICAL EXHIBITION AT PHILADELPHIA. 



309 



phragrn will cause the mirror to move, and 

 consequently the ray of light to be intermit- 

 tent upon the lampblack. Words spoken into 

 the telephone are first translated into light- 

 vibrations, and these are retranslated into 

 sound a device that manifestly may be made 

 use of in war-times. 



Alarms. Numerous devices for detecting the 

 burglar were exhibited, one of which was a 

 matting to be placed under the carpet or on 

 the floor, inside doors and windows. At night, 

 after the electric current is switched on, a step 

 upon the matting will cause the ringing of the 

 electric bell. Great numbers of annunciators 

 for hotels were also exhibited. An ingenious 

 arrangement for indicating time was shown. 

 The apparatus was meant to be used in tele- 

 phone exchanges, and was 

 so arranged that every five 

 minutes it would signal 

 the correct time indicated 

 by a clock in the central 

 station, to every subscriber 

 to that exchange. 



Cables and Wires. The 

 cable and insulated wire 

 exhibited by the Callender 

 Company were especially 

 fine. The new compound 

 used by them seems in all 

 respects to rival gutta- 

 percha, allowing them to 

 produce cables of high in- 

 sulation at a greatly re- 

 duced cost. The phosphor- 

 bronze wire, also to be seen 

 in this exhibit, deserves 

 notice, since it is coming 

 rapidly into general use. 

 Wires of this metal, be- 

 sides having greater ten- 

 sile strength, weigh about 

 one eighth as much as iron 

 wire of the same electrical 

 resistance. This admits of 

 great economy of poles, 

 both in size and number. 



The problem of under- 

 ground conductors has 

 been forced upon invent- 

 ors suddenly, by the de- 

 cision that within city lim- 

 its wires must be buried. 

 This problem is by no 

 means simple, where the 

 telegraph is concerned. It 

 is not so hard to do tbis 

 where speed may be sacrificed. The efficiency 

 of the telephone is impaired when its wires 

 lie near those of a working telegraph. It was 

 found in one instance in France, when a tele- 

 phone cable ran parallel with a telegraph cable 

 for half a mile, separated at the nearest point 

 by about 100 yards, that the telegraph message 

 could be distinctly heard in the telephone re- 

 ceiver. 



Synchronous Telegraphy. This was one of the 

 novel things at the exhibition, and attracted a 

 great deal of attention. The claims for this 

 invention were received with incredulity; but, 

 as the exhibit shows twenty-four instruments 

 telegraphing at once through one wire, the fact 

 must be accepted as proved. This result is 

 effected by taking advantage of the fact that, 

 for an appreciable though very short time, a 

 piece of wrought-iron retains its magnetism 

 after the current that has produced it stops 

 flowing in the helix around the wrought-iron 

 core, and consequently it holds the armature 

 for a moment. During this moment, a revolv- 

 ing contact has made and broken eleven other 

 connections, and come back to the same line 

 again, before the armature has been released. 



MANGIN PROJECTOR. 



This contact-wheel makes about thirty contacts 

 a second with the same wire. The receiving 

 is identical with the transmitting instrument, 

 the wheels carrying the revolving contacts 

 moving synchronously at both ends of the line. 

 This movement is regulated by two tuning-forks 

 of the same pitch, the wheels going through 

 the same arc during each swing of the tuning- 

 forks. Its effect is to make and break each 



