EXPLORATIONS OVER THE VIBRATING SURFACES OF 



TELEPHONIC DIAPHRAGAIS UNDER SIMPLE 



IMPRESSED TONES. 



By. a. E. KENNELLY and H. O. TAYLOR. 

 {Read April 22, igi3.) 



The following research was carried on, at the Massachusetts 

 Institute of Technology, under an appropriation from the American 

 Telephone & Telegraph Co. during the year 1914-1915. The ex- 

 perimental work was carried out at Pierce Hall, Harvard University. 



The object of the investigation was to explore the amplitude of 

 the small harmonic vibrations of a circular diaphragm of telephonic 

 type, clamped around the edge, and to compare the observed val- 

 ues with those which had been already deduced mathematically. 

 Hitherto, so far as we are aware, the amplitude of vibration of a 

 telephone diaphragm has been determined only at one point on 

 the surface, usually the center,^ The observations here reported 

 differ from those heretofore obtained, in extending over the entire 

 surface of the diaphragms. 



Exploring Apparatus. 

 The exploring device, or " explorer," devised and constructed 

 for this research, consists of a tiny triangular mirror fastened to a 

 little phosphor-bronze stirrup strip, and having its point applied, 

 by means of torsion in the strip, to the surface of the vibrating dia- 

 phragm at the point to be explored. The natural frequency of the 

 mirror being much greater than that impressed on the diaphragm, 

 the mirror is able to follow the vibrations of the latter, without 

 breaking out of contact. The pressure exerted by the mirror on 

 the diaphragm is so small as not materially to affect the diaphragm's 

 vibration. A beam of light, reflected from the mirror on to a trans- 

 lucent scale, was thus set into vibrations synchronous with, and 

 1 See Appended Bibliography, Nos. 2, 4, 7, 9 and 10. 



96 



