J. Trowbridge — A Telephone Relay. 341 



induction on a neighboring circuit at several points. This 

 method gets rid of mechanical pressure on the vibrating dia- 

 phragm of the telephone, and substitutes an electrical pressure 

 without any visible connection between the circuits. This 

 method also is ineffective. It has, however, a certain analogy 

 in another and more successful attempt to utilize an invisible 

 and intangible magnetic effect without bringing a mechanical 

 pressure on the telephonic diaphragm ; this method consists in 

 causing the telephonic currents to disturb a piece of iron, a 

 balanced magnet, or a suspended coil in a strong magnetic 

 field such as is found at the center of an electromagnet or 

 between the poles of a strong permanent magnet. 



The principle of this method is that of the siphon recorder, 

 the invention of Lord Kelvin, which is used on ocean cables. 

 Since the current on the cable is very feeble, and cannot work 

 ordinary telegraphic instruments, some method must be used to 

 magnify or to record the signals. The method adopted by 

 Lord Kelvin was that of a delicately suspended coil so placed 

 between the poles of a powerful magnet that when the feeble 

 currents passed through this coil it oscillated ; for the feeble 

 currents animated the coil making it an electromagnet, the 

 poles of which sought the poles of the powerful stationary 

 magnet. • Thus a very feeble electrical current could be 

 detected by the powerful magnetic influence to which it was 

 subjected. Here we have a mechanical movement, the move- 

 ment of a vibrating system, produced without the intermedia- 

 tion of visible connecting parts. The same principle has been 

 adopted in many forms of instruments for detecting and 

 measuring electrical currents, both in laboratories and in com- 

 mercial electrical installations. 



It has, therefore, occurred to many minds that by the use of 

 this principle of magnifying the vibration of moving parts by 

 the reaction between the feeble currents in such parts and the 

 environment about these parts, one should be able to strengthen 

 or repeat such vibrations. The mechanical difficulties, how- 

 ever, are very great if one endeavors to apply this principle to 

 the problem of the telephonic relay. A delicate suspension 

 such as is used in the siphon recorder or the D'Arsonval galva- 

 nometer is out of the question ; and a rigid suspension pre- 

 vents the turning movement, the seeking of the poles of the 

 powerful magnet by the little coil which conveys the feeble 

 currents. A certain measure of success, however, can be 

 obtained by careful adjustments in a laboratory ; but the utili- 

 zation of the turning movement of a little coil in a magnetic 

 field has not yet proved of commercial use in telephony. 



We are apparently brought back to some modification of the 

 simple principle of the disturbance of a powerful magnetic 



