ipis.] SURFACES OF TELEPHONIC DIAPHRAGMS. 113 



Fig. 12 is an inverted velocity-circle diagram for Diaphragm No. 

 I, based upon its measured values of m, r and s. If we take the 

 diametral velocity OM as 5 cm. per sec, with r^328 dynes per 

 cm./sec, then the vmf. which, in the particular environment of the 

 experiment, produced this velocity, would be 1,640 dynes, maximum 

 cyclic value. The particular pipe G^{'/C)2 ^), gave an observed am- 

 plitude at the diaphragm center, which, multiplied by (o = 2 tt X 792, 

 gives the line OG^ along the chord OP. The phase-angle a must be 

 obtained by considering the mechanical reactance as in (4), App. II. 

 If the vmf. of this pipe were the same as that which produced OM, 

 this point G^, would lie on the circle. Consequently, the vmf. of the 

 pipe G^ is to that of the pipe producing resonance, in the ratio 

 OGJOP. Similarly, the vmf. of the pipe G*(832'~'), is less than 

 that producing the resonant velocity, in the ratio OG^^/OR. It is evi- 

 dent that the range of any one diaphragm, for the precise comparison 

 of vmf .'s from organ-pipes of different pitch, is somewhat limited. In 

 the case presented, it would not exceed one octave, since the chords 

 far from the resonant diameter become so short. By selecting a 

 diaphragm of relatively large damping constant A=:r/2w, this 

 range can be increased. In fact, the range in w between the quad- 

 rantal points QQ' on the velocity circle, is numerically equal to v/m, 

 or twice the damping constant. 



A succession of calibrated diaphragms with overlapping ranges 

 might be employed to cover the musical scale. The writers have 

 not attempted to compare organ-pipes for standard vmf. in this 

 manner. The measurements might have to be made out-of-doors. 

 In the sound-absorbing room in which this research was carried on, 

 the effect of sound reflections from walls and other objects pre- 

 vented any standard comparisons of vmf. from being made. 



Explorations with Electromagnetically Excited Diaphragms. 



In order to ascertain the effects of exciting a steel diaphragm 

 (No. 2) electromagnetically, a No. 144 Western Electric Bell tele- 

 phone receiver was screwed into the explorer, behind the diaphragm, 

 so as to obtain the ordinary air-gap between the diaphragm and its 

 two poles. The cap or screw-cover of the ordinary telephone re- 



