290 BELL SYSTEM TECHNICAL JOURNAL 



Application of Thermistors to Control Networks.] J. H. Bollman^ and 

 J. G. Kreer.i /. R. E., Proc, V. 38, pp. 20-26, Jan., 1950. 



Abstract — In connection with the application of thermistors to regulat- 

 ing and indicating systems, there have been derived several relations be- 

 tween current, voltage, resistance, and power which determine the electrical 

 behavior of the thermistor from its various thermal and physical constants. 

 The complete differential equation describing the time behavior of a di- 

 rectly heated thermistor has been developed in a form which may be solved 

 by methods appropriate to the problem. 



Sensitive Magnetometer for Very Small Areas. "f D. M. Chapin.^ Rev. Sci. 

 Instruments, V. 20, pp. 945-946, Dec, 1949. 



Abstract — A vibrating wire system for measuring weak magnetic fields 

 is described for use in very small spaces. Quartz crystals are used for drivers 

 to get sufficient velocity with very small displacements. To adjust the 

 driving voltage to correspond exactly to the natural crystal frequency, the 

 crystal is also used to regulate the oscillator. 



Method of Calculating Hearing Loss for Speech from an Audiogram.] H. 

 Fletcher.1 Acoustical Soc. Am., Jl., V. 22, pp. 1-5, Jan., 1950. 



Abstract — The question frequently arises. Can one compute the hearing 

 loss of speech from the audiogram and thus make it unnecessary to make a 

 speech test after the hearing loss for several frequencies has been recorded. 

 This paper shows that this can be done by taking a weighted average of the 

 exponentials of the hearing loss at each frequency. Or if /3s is the hearing 

 loss for speech and j3{ the hearing loss at each frequency, 



1q(^3/io) ^ j(. ioWi«df 



The weighting factor G was determined by Fletcher and Gait from thresh- 

 old measurements of speech coming from filter systems. As specifically 

 applied to the case of hearing loss at the five frequencies 250, 500, 1000, 

 2000 and 4000 cps, the above equation is approximately equivalent to 



/Js = -10 log [.01 X \0-^^'"'^ -\- .13 X 10"^''^''°^ 



+ .40 X IQ-^''^''"^ -f .38 X lO-^'^^'^''^ + .08 X lO-^'^^''"^] 



where jSi is hearing loss at 250 cps 

 /32 is hearing loss at 500 cps 

 /Sa is hearing loss at 1000 cps 



04 is hearing loss at 2000 cps 



05 is hearing loss at 4000 cps 



t A reprint of this article may be obtained on request to the editor of the B.S.T.J. 

 1 B.T.L. 



