rr.i.F.riioNF. receiveks at /.oic roni-n inpcts 



405 



motive force of .01 ampere turns. I'or laboratory measurements on 

 this instrument a current of 2X10 '- amperes is ordinarily used, and 

 it will l)e noted that the inipedaiuc at extremely low currents is not 

 greatly different. 



It is generally known that, in the case of either a steady or an 

 alternating lield, the jK-rmeability and the shape of the hysteresis 



Fig. 3 — /, = natural frequency: A = logarithmic decrement per second; 2^ = depression 



angle of principal diameter; 2„ = maximum motional im[)edance; /f = free resistance 



at resonance; /C„ = damped impedance. 



loop for ordinary- magnetic materials reach limiting values as the 

 magnetomotive force is reduced, that is, further reductions of the 

 magnetomotive force have no effect on these magnetic characteristics. 

 The results cited above show that this condition obtains for a weak 

 alternating field when it is superimposed on a relatively strong steady 

 field. 



In the measurements of free impedance for determining the vibra- 

 tory characteristics the small sound-proof bo.\ could not be used on 

 account of the proximity of its walls. Accordingly, the receiver 

 and the impedance bridge were placed in a large sound-proof booth 

 with padded walls where the effect of reflection of sound waves was 

 very small. With the diaphragm of the instrument free to vibrate, 

 its efficiency as a sound detector was materially increased and the 

 noise in the head receivers due to the slightest movements of the 

 observer became so serious that it was not feasible to take data with 

 currents of less than 2X10"' amperes. 



