PROPERTIES AND USES OF THERMISTORS 211 



operating relays or disturbing sensitive apparatus by introducing a ther- 

 mistor in series with the circuit component which is to be protected. In 

 case of a surge, the high initial resistance of the thermistor holds the surge 

 current to a low value provided that the surge does not persist long enough 

 to overcome the thermal inertia of the thermistor. The normal operating 

 voltage, on the other hand, is applied long enough to lower the thermistor 

 resistance to a negligible value, so that a normal operating current will flow 

 after a short interval. In this way, the thermistor enables the circuit to 

 distinguish between an undesired signal of short duration and a desired 

 signal of longer duration even though the undesired impulse is several timss 

 higher in voltage than the signal. 



Oscillators, Modulators and Amplifiers 



A group of applications already explored in the laboratory but not put into 

 engineering use includes oscillators, modulators and amplifiers for the low 

 and audio-frequercy range. If a thermistor is biased at a point on the 

 negative slope portion of the steady-state volt-ampere characteristic, and 

 if a small alternating voltage is then superposed on the direct voltage, a 

 small alternating current will flow. If the thermistor has a small time con- 

 stant, T, and if the applied frequency is low enough, the alternating volt- 

 ampere characteristic will follow the steady-state curve and dV/dl will be 

 negative. As the frequency of the applied a-c voltage is increased, the 

 value of the negative resistance decreases. At some critical frequency, 

 /c , the resistance is zero and the current is 90 degrees out of phase with 

 the voltage. In the neighborhood of /c , the thermistor acts like an induc- 

 tance whose value is of the order of a henry. As the frequency is increased 

 beyor.d/c , the resistance is positive and increases steadily until it approach- 

 es the d-c value when the current and voltage are in phase. The critical 

 frequency is given approximately by 



/c = l/2r. 



If T can be made as small as 5 X 10~ seconds, fc is equal to 10,000 

 cycles per second and the thermistor would have an approximately 

 constant negative resistance up to half this frequency. Point contact 

 thermistors having such critical frequencies or even higher have been 

 made in a number of laboratories. However, none of them have been 

 made with sufficient reproducibility and constancy to be useful to the 

 engineer. It has been shown both theoretically and experimentally that 

 any negative resistance device can be used as an oscillator, a modulator, or 

 an amplifier. With further development, it seems probable that thermistors 

 will be used in these fields. 



