100 



PROCEEDINGS OF THE AMERICAN ACADEMY 



The figures obtained in these measurements lead to precisely the 

 same conclusion as those already discussed ; showing that the ampli- 

 tude of the vibration of the diaphragm produced by an alternating line 

 current of a given strength increases up to a certain point, and then 

 decreases to a very low value. The point of maximum amplitude is 

 far below the saturation limit of the magnet. It is evident, then, that 



* 2 3 ^ 3 6 7 8 9 f " 'i 'J 



it is not desirable to use an excessively strong magnet with a magneto- 

 telephone receiver, a result which agrees with the conclusions set forth 

 in both of the papers cited at the beginning of the present article. 



Our observations were not made with the direct intention of deter- 

 mining the absolute magnitude of the excursion of the telephone dia- 

 phragm, but they may throw some light upon the subject regarding 

 which the figures given by different observers differ widely. Dr. 

 C. J. Blake,* by inscribing the vibration on a plate of smoked glass, 

 obtained a value of 0.02 mm., and by the use of a micrometer screw 

 with galvanic contact a value of 0.0135 mm. Saletf employed an 



* Jour. Soc. Tel. Eng., 1878, Vol. VII. p. 247. 

 t Comptes Rendus, 1882, Vol. XCV. p. 178. 



