1915.] SURFACES OF TELEPHONIC DIAPHRAGMS. 99 



The construction of the apparatus is such, that the mirror is held 

 at all times at the center of the brass rectangular frame; while by 

 means of the two crank adjustments, the diaphragm to be explored 

 can be moved so as to bring any part of its surface beneath the 

 mirror. With the aid of the scales of distance and angle shown in 

 Fig. I, the position of the mirror with respect to the diaphragm 

 can be adjusted and read off to polar coordinates (r, 6). The 

 motion in r is controlled by the crank at the bottom, to o.i mm.; 

 while the angular motion in 6 is controlled by the crank at the side, 

 to 1°, or less if desired. The slide is held in position by flat springs, 

 attached to the rectangular frame, so as to keep the motion of the 

 slide confined to its own plane. A similar construction is used 

 with the circular frame. It is important that the plane of the dia- 

 phragm shall not be disturbed when either crank is operated. The 

 weight of the whole explorer is 4.63 kgs. (10.2 lbs). 



A magnified view of the mirror, and its stirrup frame, is shown 

 at the top of Fig. 3. The mirror, of silvered glass, about o.i mm. 

 thick, is cut in the shape of an equilateral triangle, about i mm. in 

 length of side. One vertex of the mirror is applied to the surface 

 of the diaphragm, and the mirror is fastened ■w'lih sealing wax 

 across a thin phosphor-bronze strip. This strip is approximately 

 3 mm. long between abutments, 0.02 mm. wide, and 0.013 mm. thick. 

 The weight of the mirror is about i milligram, without varnish or 

 sealing wax. Its natural frequency of vibration, as obtained pho- 

 tographically, is about 2,500 '— '. These little mirrors are apt to 

 break off the stirrup strip ; so that they have to be renewed and re- 

 calibrated occasionally. The pressure exerted on the diaphragm by 

 the point of the mirror, as measured by an auxiliary test, is approxi- 

 mately 200 dynes (204 mgm. wt.). A pressure of this order seems 

 to be desirable, so as to obtain a natural frequency of 2,500'—'. 

 If, however, explorations are confined to lower diaphragm fre- 

 quencies, the natural frequency of the explorer mirror, and its pres- 

 sure on the diaphragm, may be reduced accordingly. 



The diaphragm to be explored is 5.4 cm. in diameter, and is 

 placed in the circular frame. It is clamped tightly into this frame, 

 with the ring clamp shown in Fig. 3, which had a radius of 2.62 cm. 

 when no auxiliary clamping rings were used. The vibration explorer 



