PROPERTIES AND USES OF THERMISTORS 173 



The diameters of the beads range from 0.015 to 0.15 centimeters with wire 

 diameters ranging from 0.0025 to 0.015 centimeters. 



Rod thermistors are made by mixing the oxides with an organic binder 

 and solvent, extruding the mixture through a die, drying, cutting to length, 

 heating to drive out the binder, and sintering at a high temperature. Con- 

 tacts are applied by coating the ends with silver, gold, or platinum paste 

 as used in the ceramic art, and heating or curing the paste at a suitable 

 temperature. The diameter of the rods can ordinarily be varied from 0.080 

 to 0.64 centimeter. The length can vary from 0.15 to 5 centimeters. 

 Discs and washers are made in a similar way by pressing the bonded 

 I powders in a die. Possible disc diameters are 0.15 to ,^ or 5 centimeters; 

 |l thicknesses from 0.080 to 0.64 centimeter. 



Flakes are made by mixing the oxides with a suitable binder and solvent 



to a creamy consistency, spreading a film on a smooth glass surface, allowing 



! the film to dry, removing the film, cutting it into flakes of the desired size 



and shape, and firing the flakes at the sintering temperatures on smooth 



\ ceramic surfaces. Contacts are applied as described above. Possible 



dimensions are: thickness, 0.001 to 0.004 centimeter; length, 0.1 to 1.0 



! centimeter; width, 0.02 to 0.1 centimeter. 



! In any of these forms lead wires can be attached to the contacts by solder- 

 ' ing or by firing heavy metal pastes. The dimensional limits given above 

 , are those which have been found to be readily attainable. 



In the design of a thermistor for a specific application, the following 

 characteristics should be considered: 1) Mechanical dimensions including 

 ^ those of the supports. 2) The material from which it is made and its prop- 

 ; erties. These include the specific resistance and how it varies with tem- 

 I perature, the specific heat, density, and expansion coefficient. ^) The 

 i dissipation constant and power sensitivity. The dissipation constant is 

 I the watts that are dissipated in the thermistor divided by its temperature 

 [ rise in centigrade degrees above its surroundings.. The power sensitivity is 

 I the watts dissipated to reduce the resistance by one per cent. These con- 

 stants are determined by the area and nature of the surface, the surrounding 

 'medium, and the thermal conductivity of the supports. 4) The heat ca- 

 j.pacity which is determined by specific heat, dimensions, and density. 5) 

 :The time constant. This determines how rapidly the thermistor will heat 

 [or cool. If a thermistor is heated above its surroundings and then allowed 

 to cool, its temperature will decrease rapidly at first and then more slowly 

 until it finally reaches ambient temperature. The time constant is the time 

 ! required for the temperature to fall 63 per cent of the way toward ambient 

 i temperature. The time constant in seconds is equal to the heat capacity 

 tin joules per centigrade degree divided by the dissipation constant in watts 



