TELEPHONE DIAPHRAGMS. 



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A small triangular mirror M of very thin silvered glass, and say of 

 1 mm. in length of each edge, is fastened to a little phosphor-bronze 

 strip, about 4 mm. long, stretched between two pointed metallic 

 abutments. The strip is so fastened that one point of the mirror is 

 pressed elastically into contact with the diaphragm D, to the center 

 of which it is presented. The purpose of the mirror is to reflect a 

 beam of light, from a powerful lamp, on to a graduated scale; so that 

 vibrations of the diaphragm will cause the reflected beam to extend 

 into a band of light on the scale. Since the effective radius of lever 

 arm in the mirror is about l/3rd mm., the amplitude of motion of 

 the diaphragm can readily be magnified, say 8000 times, on a scale 



Fig. 10. — Amplitude Measurek. 



one meter distant. The ratio of the distance traversed on the scale 

 by the reflected image, to the motion of the diaphragm producing the 

 displacement, may be called the magnification factor of the instrument. 

 It depends upon the geometry of the mirror and reflecting system. 

 For any particular application, the magnification factor can be directly 

 measured by the micrometer screw-head shown at A. The brass rod 

 R, carrying the mirror on its forked extremity, moves in a guide, and 

 its movement parallel to the axis of the telephone receiver is con- 

 trolled by the screw head A. One turn of the screw head A advances 

 the mirror M towards the diapliragm through a distance of one thread. 



