444 



KENNELLY AND AFFEL. 



against the tension of the spiral spring shown. In the particular 

 screw used, there were 24 threads to the inch, or 1 turn = 1.06 mm. 

 The semicircular divided scale s enables the advance of the screw to be 

 measured to ^ degree, and estimated to jq degree; so that the ad- 

 vance could be measured to 1.5 /x, and estimated to 5 /jl. With the 

 mirror in working contact on the diaphragm at rest, the position of 

 the reflected spot of light is observed on the scale. The screw head a 

 is now turned steadily, in one and the same direction, through several 



Fio. 11. 



Photographic View of Amplitude Measurer 

 Applied to a Telephone Receiver. 



successive angles, and the corresponding excursions of the spot of 

 light recorded. Knowing the advances made by the screw, and the 

 corresponding numbers of scale divisions in the excursions, the magni- 

 fication factor for the setting is immediately deducible. A calibra- 

 tion curve for magnification factor of spot deflection, versus-screw 

 head angular motion, usually follows a straight line, over sa}' 60 

 microns of diaphragm amplitude; provided that the range of scale 

 deflection does not commence with the mirror almost breaking contact 



