166 LAWRENCE R. PROUTY AND JAMES D. HARDY 



example, undergoes marked changes. A common laboratory sub- 

 stance, Rochelle salt crystals, exhibits a large shift in dielectric con- 

 stant between 20 and 30° C. but is destroyed by temperatures over 

 105°. It is somewhat difficult to use these substances for tempera- 

 ture determination because resistance losses are large enough to re- 

 quire separate balancing for a null reading if a bridge circuit is em- 

 ployed to measure the change in capacitance. The dielectric con- 

 stant shift can be used to better advantage to measure temperature if 

 these substances are used as the capacitance part of an inductance- 

 capacitance circuit or phase shift oscillator. 



3. Thermosensitive Magnetic Alloys 



Thermosensitive alloys of silicon steel, chromium, and nickel can 

 be made with Curie points {T^, point at which magnetization drops 

 to zero) below 100°C. with rapid decrease in magnetization starting 

 at a temperature value 60% of the Curie point. A coil using a core 

 of one of these alloys can be employed with an audio-frequency circuit 

 to measure temperature within the range of 60% (T/Tc) of the Curie 

 point of that alloy. 



4. Thermal Conductivity Cells 



Temperature-resistance relationships in wires heated by a con- 

 stant current or a constant terminal voltage and placed in glass cells 

 are used to measure pressure, humidity (electric hygrometers), radia- 

 tion, and temperature and to analyze gases. By determination of 

 the rate of heat dissipation from the wire to the surrounding medium, 

 a coil of fine platinum or tungsten wire may be used as a flow meter 

 for either liquids or gases. 



The thermoconductivity principle is also frequently applied in 

 studies of human and animal physiology. The rate of dissipation of 

 the heat in the intestine, for instance, may be determined by resist- 

 ance changes in a heated wire enclosed within the walls of a balloon 

 placed in the intestine {32) . Similarly, a heated wire lead into a blood 

 vessel through a hypodermic needle may be used to yield accurate 

 determinations of the rate of flow of blood. 



5. Bimetallic Strip Thermometers 



Laminated strips of metals that have different temperature 

 coefficients of linear expansion are widely used to determine tem- 



