PROPERTIES AND USES OF THERMISTORS 



189 



where H = heat capacity m joules per centigrade degree. The solution of 

 this equation is 



(r - r„) = (r„ - r„) 



in which 2\ — T when / = and 



r = H/C, 



(20) 



(21) 



where r is in seconds. It is commonly called the time constant. From 

 equation (20) it follows that a plot of log {T — T a) versus t should yield a 

 straight line whose slope = — 1/2.303t. If // and C vary slightly with 

 temperature then t will vary slightly with T and /. The line will not be 

 perfectly straight but its slope at any t or (T — To) will yield the appro- 



Table I. — Values of C, t, H as Functions of T for a Thermistor of Material No. 1 



ABOUT 1.2 Centimeters Long, 0.30 Centimeters in Diameter and Weighing 0.380 Gram 



Ta = 24 degrees centigrade 



priate t or H/C for that T. As previously described, C can be determined 

 from a plot of watts dissipated versus T. For this thermistor this curve 

 became steeper at the higher temperatures so that C increased for higher 

 temperatures. Table I summarizes the values of C, r, and // at various T 

 for the unit in air. 



When a thermistor is heated by passing current through it, conditions 

 are somewhat more involved since the PR power will be a function of time. 

 At any time in the lieating cycle the heat power liberated will be equal to 

 the watts dissipated or C{T — Ta) plus watts required to raise the tem- 

 perature or HdT/dl. The heat power liberated will de})end on the circuit 

 conditions. In a circuit like that shown in the upper corner of Figure 14, the 

 current varies with time as shown by the six curves for six values of the 

 battery voltage E. If a relay in the circuit operates when the current 

 reaches a definite value, a considerable range of time delays can be achieved. 



