I9I5-] 



SURFACES OF TELEPHONIC DIAPHRAGMS. 



103 



(i) The acoustic vibrations were supplied from one of a series 

 of small organ-pipes, giving fairly simple musical tones between Co 

 of 128 '—', and Cq of 2,048 '--'. The organ-pipe selected was mounted 

 vertically in a block on the table, at the back of the vibration ex- 

 plorer, and supplied with air at constant pressure (about 18 cm. of 

 water) from a pneumatic tank. The whole apparatus was placed 

 inside a sound-damping wooden-frame booth (274 cm. X 183 cm. X 

 214 cm. high), lined on the inside with hair-felt, 2.5 cm. thick, sur- 

 faced with thin cloth. The observer, after turning on the air to the 

 organ-pipe, observed the amplitude of the luminous band on the 

 translucent screen F, Figs. 4 to 6, as the mirror was applied to 

 different successive points on the diaphragm. 



(2) The clamping ring of the diaphragm in the explorer was 

 chosen of such dimensions that a standard telephone receiver could 

 be substituted for it. In this case, a steel diaphragm had to be 

 employed. The telephone was then operated by a feeble measured 

 alternating current (2.0 milliamperes) obtained from a Vreeland 

 mercury-arc oscillator having a frequency adjustable, by successive 

 steps, between 430-^ and 2,500^— >.^ 



Exploration with Diaphragm No. i. 



Diaphragm No. i was a telephone-receiver diaphragm of steel, 



japanned on one side. Its dimensions are given in Table III. The 



diaphragm was clamped, around the boundary, between opposing 



circular knife-edges. 



TABLE I. 



Vibration Amplitudes over Diaphragm No. i, at Frequency 608^ — ', for 

 Nine Different Azimuths 6, and Seven Different Radial Distances r. 



2 See Bibliography No. 5. 



