3-46 J. Trowbridge — A Telephone Relay. 



have relayed loud and articulate speech. The main features 

 of this relay are these : the magnetic field acts both in the 

 direction towards B as well as toward A. 



Fig. 2 shows by the lines of magnetic filings the character of 

 this magnetic field. This arrangement may be considered a 

 modification of the principle of the siphon recorder of Lord 

 Kelvin, in which a to and fro thrust or vibration takes the 

 place of the rotating or turning effect of feeble currents actu- 

 ating a small coil suspended in a strong magnetic field. 



The second important principle is the freedom of the cen- 

 tral portion of the diaphragm A from the pressure of the 

 transmitter T. The third point is the transmission of the 

 vibrations of this diaphragm A by means of the sound-trans- 

 mitting support of the transmitter to the transmitter T. As I 

 have already said, the air enclosed in the chamber DE plays a 

 very small part in the transmission of the vibrations of the 

 diaphragm A. A proof of this important fact is as follows. 



Fig. 3 represents a front view of the support DE of the 

 transmitter, which in this case consisted merely of a metallic 

 bar. Fig. 4 is a side view of this case. Here the air space 

 between the transmitter and the diaphragm A is not enclosed, 

 being open on both sides of the bar, which is connected at its 

 edges w T ith the diaphragm A. The transmission of speech in 

 this case is loud, while if the connection at D and E with the 

 diaphragm are removed and the bar upon which the trans- 

 mitter rests in the same position parallel to the diaphragm A, 

 a very feeble sound is transmitted through the intervening air. 



One can, therefore, adjust the transmitter without bringing 

 a pressure upon the most sensitive portion of the diaphragm 

 A, its center, and without disturbing the adjustment of the 

 moving coil C. The relay is very sensitive ; it is loud, and 

 the articulation good. Moreover it is free from the- objection- 

 able growling or enhancing effect of the crepitation of the 

 microphonic contacts on the vibrations of the magnetic portion 

 of the relay. I have been much indebted to the suggestiveness 

 and mechanical skill of the mechanician of the laboratory, 

 Mr. George W. Thompson. 



The engineering problems connected with loading telephone 

 lines are most interesting, and when completely solved may 

 obviate the use of relays, and telephonic engineering may go 

 through a phase analogous to that of the Morse telegraph, in 

 which as I have said the telegraphic relay is subordinate in 

 importance to good conducting lines. Nevertheless we have 

 not reached this point yet, and the cost of a relay is practically 

 nothing in comparison with the enormous expense of a loaded 

 line. The relay, described in this article, seems to me to be 

 an important solution of the problem of the telephonic relay. 



Jefferson Physical Laboratory, 

 Harvard University. 



