272 PROCEEDINGS OP THE AMERICAN ACADEMY. 



i? T is a resistance equal to that of one coil of the telephone and 



of negligible inductance. Adjustment is to be made until with the 



same value of t A the telephone is silent when in place, as shown, and 



E 

 when interchanged with E T then R x = -=. 6 is given by micrometer 



readings as before. 



(e) We may also use the differential telephone to indicate when an 

 unknown inductance is neutralized by a condenser of known variable 

 capacity, by placing one of its coils in series with the inductance and 

 condenser, and the other in series with a variable inductionless resist- 

 ance i? l5 shunted around the above condenser, inductance, and tele- 

 phone coil, if the resistance E 1 and capacity C be varied until silence 



is obtained. Then L = -tt— and E = i?i. It will be desirable 



Or- 



to employ an air-condenser to eliminate absorption effects, though 



this procedure has the disadvantage of requiring a very bulky and 



costly condenser when small inductances are to be measured. 



With this apparatus we may also determine the electrostatic ca- 

 pacity of a condenser when not complicated by absorption effects. 

 For instance, by the first method, we should insert in series with the 

 condenser an inductionless resistance of such a magnitude that the 

 angle of advance could be determined with accuracy. 



Then 



I 1 1 



E~ J i 



tan * = CEP' 



7 V 



C = -FPn—- — r ' and R — 1 cos V'- 



EP sin ^ I 



HE = 0, * = 90° and = -^- 



The above are the chief uses to which the apparatus may be put. 



We hope in the near future to investigate the transmission of tele- 

 phonic and telegraphic signals, and to study various small induction 

 coils such as are used in telephony, as well as the errors introduced 

 into results by the above methods owing to lack of fulfilment of the 

 assumed law by the E. M. F. 



Rogers Laboratory of Physics, 

 September, 1893. 



