PROPERTIES AND USES OF THERMISTORS 207 



shape or nonlinear distortion. Speech volume limiters of this type can ac- 

 commodate large volume changes without producing wave form distor- 

 tion. i^.i^ 



Remote Control Swiches 



The contactless switch and rheostat are natural extensions of the uses 

 just discussed. The thermistor is used as an element in the circuit which is 

 to be controlled, while the thermistor resistance value is in turn dependent 

 upon the energy dissipated directly or indirectly in it by the controlling cir- 

 cuit. By taking advantage of the nonlinearity of the static volt-ampere 

 characteristic, it is possible to provide snap and lock-in action in some 

 applications. 



Manometer 



Several interesting and useful applications such as vacuum gauges, gas 

 analyzers, flowmeters, thermal conductivity meters and liquid level gauges 

 of high sensitivity and low operating temperature are based upon the 

 physical principle that the dissipation constant of the thermistor depends 

 on the thermal conductivity of the medium in which it is immersed. As 

 shown in Fig. 10, a change in this constant shifts the position of the static 

 characteristic with respect to the axes. In these applications, the unde- 

 sired response of the thermistor to the ambient temperature of the medium 

 can in many cases be eliminated or reduced by introducing a second thermis- 

 tor of similar characteristics into the measuring circuit. The compensating 

 thermistor is subjected to the same ambient temperature, but is shielded 

 from theeflfect being measured, such as gas pressure or flow. Thetwo therm- 

 istors can be connected into adjacent arms of a Wheatstone bridge which 

 is balanced when the test effect is zero and becomes unbalanced when the 

 effective thermal conductivity of the medium is increased. In gas flow 

 measurements, the minimum measurable velocity is limited, as in all '*hot 

 wire" devices, by the convection currents produced by the heated thermistor. 



The vacuum gauge or manometer which is typical of these appHcations 

 will be described somewhat in detail. The sensitive element of the thermis- 

 tor manometer is a small glass coated bead 0.02 inch in diameter, suspended 

 by two fine wire leads in a tubular bulb for attachment to the chamber whose 

 gas pressure is to be measured. The volt-ampere characteristics of a typical 

 laboratory model manometer are shown in Fig. 24 for air at several absolute 

 pressures from 10~® millimeters of mercury to atmospheric. The operating 

 point is in general to the right of the peak of these curves. Electrically 

 this element is connected into a unity ratio arm Wheatstone bridge with a 

 similar but evacuated thermistor in an adjacent arm as shown in the circuit 



