an Electric Micrometer. 561. 



From the data obtained an attempt was made to draw up 

 roughly a scale of loudness, thus : — 



(1) Begins to be audible 0*4 fifj. 



(2) ,, „ comfortably loud 50 „ 



(3) „ „ uncomfortably loud 1000 ,, 



(4) ,, „ overpowering 5000 „ 



We need standards of loudness corresponding to our system 

 of standards of light-intensity. (2), (o), and (4) are only 

 meant to be rough indications, but (1) (which is a direct test 

 of the limit of hearing and does not involve the observer's 

 personal judgment of loudness) is much more important. 

 The values for it apree well together *. 



(4) The Damping of the Diaphragm. 



In Section III. a measurement is made of sound-impulses. 

 It is interesting, therefore, to know the nature of these 

 impulses, i.e. the extent to which the vibrations are damped. 



We proceed thus : — (1) Measure the position (a) of the 

 contact of the telephone when no current is passing in it. 

 (2) Pass a current in the telephone so as to draw the 

 diaphragm away from contact, then measure the new contact- 

 position (a ). (3) Draw away the contacts, and then stop 

 the current ; the diaphragm will be released and will just 

 make contact if we arrange the pointer aright ; measure this 

 contact-position (a^. 



It is to be understood that a and a, and a n are displace- 

 ments from the normal position a ; then — =p, is the 

 damping factor for one ^ vibration. a ° 



For a few vibrations it seems fair to assume for our pur- 

 poses that p does not alter much. 



Then we have for the position after n half-vibrations, 



w - ao=a - 



* A. Franke (see Proc. Soc. Tel. Eng. vol. xvi.) made a determination of 

 the amplitude of a diaphragm for the smallest sound audible. The value 

 obtained was 1*2 /x/x. But whereas it refers to a continuous sound, the 

 value obtained by the author refers to a sound-impulse. The method of 

 Franke was au interference optical one, and the smallest measured dis- 

 placement was 52 /x/x ; also he seems to have trusted to having a straight 

 line from that point to the origin. 



Cross \ T Mansfield (Proc. Amer. Acad. vol. xx. 1893) summarize the 

 work which had been done on this value. 



