PROPERTIES AND USES OF THERMISTORS 



209 



permits measurement over wider ranges of pressure. Special models have 

 also been made for use in corrosive gases. These expose only glass and plati- 

 num alloy to the gas under test. 



Timing Devices 



The numerically greatest application for thermistors in the communication 

 field has been for time delay purposes. The physical basis for this use has 



4 6 6|0 



2 4 6 810-2 2 4 6 B|0- 



PRESSURE IN MM OF MERCURY 



6 8 I 



Fig. 25. — Operating circuit and calibration for a vacuum gauge utilizing the thermistor 

 of Figure 24. 



been discussed in Part I for the case of a directly heated thermistor placed 

 in series with a voltage source and a load to delay the current rise after 

 circuit closure. This type of operation will be termed the power driven 

 time delay. 



By the use of a thermistor suited to the circuit and operating conditions, 

 power driven time delays can be produced from a few milliseconds to the 

 order of a few minutes. Thermistors of this sort have the advantage of 

 small size, light weight, ruggedness, indefinitely long life and absence of 

 contacts, moving parts, or pneumatic orifices which require maintenance 



