ipis-] 



SURFACES OF TELEPHONIC DIAPHRAGMS. 



105 



number of nodal circles, and also according to the number of nodal 

 diameters present.^ 



A similar exploration was made over the diaphragm, with 

 acoustic excitation from an organ-pipe giving Q (2,100 -— '). Here 

 the points of observation were in steps of about 3.3 mm. in r, and in 

 steps of 40° in 0, as before, with magnification-factor, M= 1,265. 



TABLE II. 



Vibration Amplitudes over Diaphragm No. i, at Frequency 2,100^ — ■, for 



Nine Different Azimuths 6, and Seven Different Radial Distances 



r, Five Only Giving Readable Deflections. 



The vibration contour-lines for this case are given in Fig. 8. 

 Here again it is seen that, setting aside irregularities in the dia- 

 phragm, and allowing for errors of observation (which are more 

 noticeable with the small amplitudes of higher pitch), the mode of 

 vibration is essentially fundamental, since there are no perceptible 

 nodal circles or nodal diameters. 



Having thus ascertained that both at pitch D\ (608 ^~'), and at 

 (76(2,048'—), the first mode of vibration was presented, a series 

 of careful explorations were made at a number of intermediate 

 pitches. These likewise all showed the first or fundamental mode 

 of vibration. See Table WA. 



Observations were also made, at organ-pipe frequencies down to 

 128 '— '. Explorations would be very difficult to obtain on this 

 diaphragm at such low frequencies, owing to the small vibration 

 amplitudes produced ; but the indications were that the fundamental 

 mode of vibration was maintained throughout. 



3 See Appendix i. 



PROC. AMER. PHIL. SOC, LIV, 2l7. H, PRINTED JULY 6, I915. 



