248 PSOCEEDINGS OP THE AMERICAN ACADEMY 



XII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXIII.— MEASUREMENT OF THE STRENGTH OF 

 TELEPHONE CURRENTS. 



By Charles R. Cross axd James Page. 



Communicated October 14, 1SS5. 



So far as is known to the authors of the present paper, no measure- 

 ments have ever been made of the strength of such telephone currents 

 as are actually used in the transmission of speech. The few figures 

 that have been given are estimates rather than measurements. Some 

 very interesting results are given by Dr. C. J. Blake (Jour. Soc Tel. 

 Engineers, 1878, p. 247) regarding the logographic values of the 

 different vowels as determined by the throw of the needle of a mirror 

 galvanometer used in connection with a magneto transmitter. This 

 logographic effect, however, should be carefully distinguished from that 

 of the alternating currents utilized in the continuous transmission of 

 sound, as, even with apparatus suitable for the measurement of such 

 currents, it is liable to mask the phenomena to be studied vmless care 

 be taken to avoid such disturbing effects. 



The lack of figures in so interesting and important a subject as the 

 one under consideration arises from the fact that an electro-dyna- 

 mometer of some form must be used for the necessary measurements, 

 and an exceedingly sensitive instrument is essential in order to obtain 

 sufficient deflection to give results of any value. 



A form of electrical balance similar to that figured by Maxwell 

 (Electricity and Magnetism, 2d ed., Vol. II. p. 342) was at first con- 

 structed, but this failed to give satisfactory results. Subsequently, 

 success was obtained by using a Kohlrausch unifilar electro-dyna- 

 mometer, made by Hartmann, of the form described in Wiedemann's 

 Annalen, Vol. XV., 1882, p. 550. The instrument was modified by 

 removing the heavy movable coil furnished by the maker, which was 

 of altogether too low resistance for our purpose, and replacing it by a 



