643 



per sec. lliat is a power of 0,83 >( lO-i^ watts, in order to produce 

 a sensation of sound; tlie necessary amplitude of tlie air waves 

 being a tlionsand times smaller than the diameter of a molecule. 

 In accordance with this the telephone is capable of responding 

 audibly to very weak currents. The modern telephones, now much 

 in use in wireless telegraphy, which ai'e put in tune with the most 

 favourable note for perception by the human ear, are to be consi- 

 dered among the most sensitive of all existing instruments. 



Max Wien ') states thai for the most sensitive tele[)hone under 

 most favourable conditions a power of 3,03 JOi* watts is wanted 

 to produce a just barely audible sound. Austin ^) indicates a 60 

 times smaller value viz. 0,5 X 10~i-^ watts. 



To evaluate the sensitiveness of the galvanometer we suppose 

 that during the reception of a signal a uniform etfective electro- 

 motive force E be applied to the terminals of the string. At the 

 llrsi moment when the sti'ing is still in rest, it will be travei'sed 



by a maximum current / =r ——, where H' repiesents the ohmic resist- 



ance of the string, but gradually the curient will decrease by the 

 back-electromotive force which is set up in llie string by its movement. 

 If we neglect the internal friction in the string itself and make the 

 vacuum so high, that it may be considered as absolute, the back- 

 electromotive force produced will be equal to E as soon as the 

 end- amplitude is attained. The cnrrenl flowing through the string 

 then = 0. As long as the signal lasts the string goes on oscillating 

 in tune with it without consuming energy. 



For evaluating the sensitiveness we have to take the maximum 



E' 

 number ot watts wanted i.e. ~ It the string have a mass of M 



grams, being in tune with zV cycles per sec, and its electromagnetic 

 decrement being rf^,,,, the number of watts wanted to induce an 

 end-amplitude of U cetim. is 



B = -'^- . 10-^ XMU' N'a,,„ (4) 



Li 



or also 



4500 .t' J/f-'' 



1) Wiedemann's Annalen 4. IV, 1901, p. 450. 



-) Jahrbuch der ciraiitl Telegr. 11 and 12, 1916. „Conf. also H. 0. Taylor. 

 Telephone receivers and radio telegraphy. Proceedings of the institute of Radio 

 engineers, 1918, Vol. 6, p. 37. 



42* 



