84 Professor J. A. Fleming [March 27, 



WEEKLY EVENING MEETING, 

 Friday, March 27, 1914. 



The Rioht Hon. Sir James Stirling, P.O. M.A. LL.D. F.R.S., 

 Vice-President, in the Chair. 



Professor J. A. Fleming, M.A. D.Sc. F.R.S. M.R.L 

 Improvements in Long-Distance Telephony. 



Great advances have been made of late years both in the scientific 

 theory and in the practical appliances of telephony. To a few, but 

 onlv to a few, of these improvements can your attention be invited 

 during the present hour. 



When we speak to an ordinary telephone transmitter, the voice 

 causes rapid changes of air pressure against a thin metal disc called 

 the diaphragm. This disc is bent inwards or outwards to a small 

 extent in a manner which has a certain correspondence with the 

 changes of air pressure. 



These motions of the diaphragm are caused to compress more or 

 less some granulations of hard carbon contained in a shallow recep- 

 tacle behind it, and this again varies the electrical resistance in a 

 circuit of which the carbon forms a part. 



These changes in conductivity in turn produce variations of a 

 nearly similar character in an electric current flowing in the line Avire. 



At the receiving end of the line this current flows through an 

 induction coil and creates a secondary current, and the latter, circu- 

 lating in the coils wound on the polar extensions of the electro- 

 magnet of the P)ell telephone receiver, sets in motion its diaphragm 

 and so reproduces approximately the changes of air pressure, and 

 therefore the sound made against the transmitter. 



Between speaker's voice and listener's ear there are therefore 

 many transformations of energy, and at each of these there is not 

 only some dissipation of energy, but also an imperfect translation of 

 the complex motions or currents. 



The transmitter diaphragm does not exactly copy by its motion 

 the changes of air pressure made by the voice near it. The current 

 into the line does not vary precisely in accordance Avith the motions 

 of the diaphragm. The current at the receiving end does not 

 accurately resemble that at the sending end, neither does the receiver 

 translate l)ack these received current variations into changes of air 

 pressure or motion without error. 



Apart altogether from the complications involved in working an 

 Exchange System into which a human, and therefore a falUble, element 



