PROPERTIES AND USES OF THERMISTORS 



185 



tor and its surroundings. The value of C for a particular thermistor in 

 given surroundings can readily be determined from the V versus / curve in 

 either Figs. 8 or 9. For each point, V/I is the resistance while V times / 

 is IF, the watts dissipated. The resistance data are converted to tempera- 

 ture from R versus T given by equation (2). A plot is then made of W 

 versus T. For thermistors in which most of the heat is conducted away, 

 W will increase linearly with T, so that C is constant. For thermistors 

 suspended by fine wires in a vacuum, W will increase more rapidly than pro- 

 portional to T, and C will increase with T. For thermistors of ordinary 

 size and shape, in still air, C/Area = 1 to 40 milliwatts per centigrade degree 

 per square centimeter depending upon the size and shape factor. 



B=3900 



C=5X10 WATTS/DEG. 



aoo'K 



100 



10"* 10-** 10"' 10-^ 10"' I K) 



CURRENT IN AMPERES 



Fig. 11.— Logarithmic plots of voltage versus current for three values of the resistance, 

 Ro , at ambient temperature. These curves are calculated for the constants given in the 

 upper part of the figure. 



The user of a thermistor may want to know how many watts can be dis- 

 sipated before the resistance decreases by one per cent. This may be called 

 the power sensitivity. It is equal to C/{a X 100), and amounts to about 

 one to ten milliwatts per square centimeter of area in still air. Both C and 

 the power sensitivity increase with air velocity. The dependence of C on 

 gas pressure and velocity is the basis of the use of thermistors as manom- 

 eters and as anemometers or flowmeters. Note that in Fig. 10 one curve 

 can be superposed on any other by a shift along a constant resistance line. 



Figure 1 1 shows a family of log V versus log / curves for various values on 

 Ro while B, C, and To are kept constant. This can be brought about by 

 changing the length, width and thickness to vary Ro while the surface area 

 is kept constant. If the resistance had been changed by changing the am- 

 bient temperature. To , the resulting curves would not appear very different 



