432 KENNELLT AND AFFEL. 



Direct Measurement of a and s. 



The direct measurement of both A and s, without reference to a 

 circle diagram, was described by Abraham,^ whose theory, for the 

 particular case investigated, was limited to the use of these two con- 

 stants only. 



The direct measurement of a involves the determination of the pull 

 on the diaphragm, excited by a known feeble continuous-current 

 strength. It is doubtful whether satisfactory close approximations 

 to the value of a can be made by continuous-current excitations; 

 since alternating-current excitations are actually involved. With an 

 exciting current strength of the same order of magnitude as is available 

 with alternating-currents, the amount of diaphragm displacement is 

 exceeding small, usually less than half of one micron;^ whereas with 

 a. c. excitation, at the resonant frequency, the displacements are com- 

 monly increased 30 times, and may be increased more than 50 times. 

 The direct measurement of a is thus difficult, and subject to correction, 

 for the change from continuous to alternating magnetic fluxes. 



Direct Measurement of s. 



Theoretically, the measurement of the elastic constant s of a dia- 

 phragm, in observing the central displacements produced by centrally 

 impressed forces, appears both simple and promising. This measure- 

 ment appears to have been first carried out by Abraham, in the pub- 

 lication above referred to; although the method is not disclosed. 

 Attempts were made in the research here reported to measure s by 

 means of the apparatus shown in Figure 8. The receiver T is mounted 

 vertically, and the lever ab, fulcromed at F, and maintained in equilib- 

 rium by means of the counterpoise c, when the weight w is absent. A 

 blunt point, on the loAver side of this lever, is pressed down against 

 the surface of the diaphragm, when the known weight w is applied. 

 The force, in dynes, impressed on the center of the diaphragm is thus 

 known. The point p of the micrometer screw-head M, comes into 

 contact with the top of the lever ab, the moment of contact being 

 determined electrically by the head telephones H, in the circuit i. 



5 Bibliography, No. 6. 



6 1 micron, commonly represented by 1 ju = 10"*^ meter = 10~^ mm. 



