J. Trowbridge — A Telephone Relay. 343 



In this form of telephonic relay we, therefore, have a micro- 

 phonic contact connected with what may be called an iron 

 plunger, the position of which in a magnetic field is modified 

 by changes in its magnetism by surrounding feeble telephonic 

 currents. The only mechanical connection is that of this mov- 

 ing plunger with the microphonic contact. 



The magnetic part of a relay embodying the above ideas 

 can be suitably constructed so as to perform their part to a 

 commercial degree of perfection ; the principal imperfection 

 of the relay arises from the microphonic part. Among such 

 imperfections the most notable one is the roaring or " growl- 

 ing " of the microphone when a strong battery is used to get 

 the greatest degree of sensitiveness from it. This noise, which 

 arises in great part from crepitations produced by heat, can 

 completely overpower telephonic transmission of speech. 

 This crepitation is greatly enhanced by the direct connection 

 of the plunger or moving electromagnetic coil with the micro- 

 phonic contact ; for the movements in the magnetic field and 

 the crepitations in the microphone get into a swing together, 

 mutually aiding each other. This mutual action is one of the 

 greatest barriers to the perfection of a telephonic relay in 

 which a close connection exists between the parts moving in 

 the magnetic field and the microphonic contacts. To over- 

 come this defect would be a great service to the art of tele- 

 phony. 



It is thought by some that this crepitation noise is analo- 

 gous to that of the singing electric light arc ; it can be 

 started in any transmitter either by too strong a current or by 

 a suitable reaction between the vibratory motions in the trans- 

 mitter and the current in the telephone circuit of which the 

 microphone is a part, for instance the roaring can be started by 

 holding a telephone connected with the circuit directly in 

 front of the transmitter. 



With the view of accomplishing two results, I have constructed 

 the following form of telephonic relay- These results to be 

 attained are as follows : First, the prevention of the reaction of 

 the magnetic parts on the crepitation of the microphone to 

 obviate the excessive roaring or " growling" ; secondly, to pro- 

 vide means for a separate adjustment of the magnetic moving 

 parts and the transmitter. This separate adjustment is evi- 

 dently highly desirable ; for the part moving in the magnetic 

 field may be in a suitable position for the greatest sensitiveness, 

 while the microphonic contact has not a suitable contact pres- 

 sure ; and any disturbance of adjustment of one of these 

 parts of the apparatus disturbs the other. In accomplishing 

 these results, the fact came out with great clearness that the 

 separation of the microphonic contact by means of an air 



