OF ARTS AND SCIENCES. 101 



optical method similar to that employed by Fizeau for measuring the 

 expansion of solids, based upon the variation in position of Newton's 

 rings when these were formed between a light piece of glass carried 

 by the telephone disk and a fixed disk of the same material placed in 

 front of it. The excursion as thus measured was from 0.0002 mm. to 

 0.0003 mm. Frohlich* measured the excursion by means of a beam 

 of light reflected from a mirror carried by the diaphragm, the beam 

 being cast upon a screen and its motion measured when the receiver 

 was operated. He gives 0.035 mm. as the value of the amplitude of the 

 motion of the diaphragm. Frankef employed an optical method simi- 

 lar to that used by Salet, making the assumption that the amplitude of 

 the vibration is proportional to the strength of the current, a supposi- 

 tion, however, which our own results, as given in the preceding pages, 

 do not fully bear out; he concludes that the excursion of the diaphragm 

 for a sound which is just audible is less than 1.2 X 10"" mm. 



Some of these differences are doubtless due to the different trans- 

 mitters used. Blake employed a box magneto-transmitter, Salet a 

 hand magneto, and Frohlich a microphone. Also it is very possible 

 that in some of the methods there is more or less interference with the 

 free motion of the diaphragm. The following considerations may there- 

 fore be of interest, although they do not lead to absolutely conclusive 

 results. 



In all of our experiments, the line current had a value considerably 

 in excess of even a strong telephone current. The weakest value of 

 the line current employed was five milliamperes, while even two milli- 

 amperes is a large value for a telephone current produced by a power- 

 ful Running microphone transmitter. Hence the actual values of the 

 excursions measured by us are much larger than those assumed by 

 the diaphragm of the telephone receiver in practice. We have, how- 

 ever, endeavored to calculate this approximately from the data at 

 hand. It is clear that, if we can obtain the equation of one of the 

 curves in Figure 1, which corresponds to a strength of field of the mag- 

 nitude employed in the ordinary telephone receiver, we may from this 

 obtain the desired result by substituting in this equation the value of 

 the telephone current. A study of the curves shows that they are all 

 approximately parabolas with the equation y^ = mx^, n being greater 

 than 2, but the values of m and n are different for the different curves. 

 It was necessary therefore to determine by experiment the strength of 



* La Lumiere Electrique, 1887, Vol. XXV. p. 180. 

 t Elektrot-echnisclie Zeitschrift, 1890, Vol. XI. p. 288. 



