an Electric Micrometer. 553 



(3) Diaphragm Movement. 



There are two distinct measurements involved : — 



A. Observe the position of the diaphragm at rest by making 

 the contact ah (fig. 2, p. 543), then draw away a, and pass a 

 steady current through the receiver so as to move the diaphragm 

 to a new position of rest; now move up a slowly towards the dia- 

 phragm, watching the graduated disk and listening for contact. 

 Thus we can measure the movement of the diaphragm centre 

 due to a steady current. This we can do for any current down 

 to such small ones as cause a movement too minute for obser- 

 vation ; then we shall have a relation between current in the 

 telephone, and diaphragm displacements, between any desired 

 limits of the current. Plot a curve for this relation. 



B. Unscrew the telephone holder from the apparatus, fit 

 on an earpiece to it, and place the receiver in a circuit con- 

 taining also a cell (whose E.M.F. is known), a resistance-box 

 and a tapping-key, the receiver being shunted by another 

 resistance-box. 



If the ear is placed on the receiver when the key is re- 

 leased a sound is heard ; this can be reduced in intensity by 

 increasing resistance, or decreasing the shunt, until no sound 

 is heard. Find the limit ; this gives us the lower limit of 

 audibility of sounds produced in this particular way as impulses. 

 Observe the current used corresponding to the limit. 



Now return to the curve obtained in A, and mark on it, by 

 extrapolation, the current just found. This will give us the 

 movement corresponding to the smallest audible sound 

 impulse. 



It must be observed that we are dealing with an impulse, 

 for the diaphragm is released from a position of strain, 

 vibrates for some time under considerable damping, and so 

 soon comes to rest. In Section IV. an attempt is made to 

 represent roughly the nature of the impulse in this special case. 



The diaphragm in its vibration has its own particular 

 frequency, and our determination refers to this frequency. 



In all cases 1 measured my own limit of hearing (for both 

 ears) so that the value obtained is probably somewhat low, 

 as 1 have had considerable practice in listening to small 

 telephone sounds. 



So faint is the smallest sound that the observations were 

 onlv possible in the dead of nights 



Tables VI A., A 7 IIa., VIII a. give four complete sets of 

 diaphragm readings. 



Tables VIb., VII b., VIII B. give the corresponding currents 

 for smallest sound-impulses (p. 554). 



Phil. Mag. S. 5. Vol. 50. No. 307. Dec. 1900. 2 K 



