110 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



Fig. (1-5)8 

 jauge. 



Circuit diagram of a Pirani 



gas molecules collide with it. As the pressure is lowered, the wire 

 will lose heat less rapidly because there are fewer molecules available 

 for heat transfer; consequently its temperature will increase. 



Cooling of the wire is the result not only of thermal conduction 

 through the gas; radiation as well as conduction through the sup- 

 ports of the wire also play a part which increases in relative signifi- 

 cance as the pressure is lowered. At less than 10~ 3 mm the loss by 



radiation and conduction through 

 the supports begins to get larger 

 than the heat loss by conduction 

 through the gas. 



The temperature of the filament 

 can be measured in two ways: 

 Either a thermoelement is hooked 

 on the filament and furnishes 

 an output proportional to the 

 filament temperature, as in the 

 thermo-couple gauge, Fig. (l-5)7a, 

 or the filament is made of a material with a high resistance- 

 temperature coefficient (tungsten, platinum, nickel) and the variation 

 of resistance caused by a variation of the filament temperature is 

 measured, as in the Pirani gauge, Fig. (l-5)76. x 



The Pirani gauge is commonly connected in a resistance bridge as 

 shown in Fig. (1-5)8. The voltage or the current applied to the bridge 

 is kept constant and at a given pressure in the gauge the bridge is 

 balanced by adjusting R v i? 3 , or R 4 . A variation of the pressure 

 causes a bridge unbalance, i.e., the appearance of a voltage between 

 A and B. 



Another way of operating the gauge is the following : The bridge 

 is continuously kept in balance. If the filament temperature changes 

 in response to pressure variation in the gauge, such a change is com- 

 pensated for by varying the current through the filament (adjust- 

 ment of the resistance R s ). The change of current or voltage applied 

 to the bridge or to the filament is noted and is a measure for the gas 

 pressure surrounding the filament. 2 A feedback-controlled, self- 

 balancing thermal-gauge system is described by Leek and Martin. 3 

 With this method the temperature of the gauge can be kept quite low 

 (less than 75°C). 



1 M. Pirani and R. Neumann, Electronic Eng., 17, 227 (December, 1944); 

 ibid., 322 (January, 1945); ibid., 367 (February, 1945); ibid., 422 (March, 1945). 

 2 N. R. Campbell, Proc. Phys. Soc. (London), 33, 287 (1921). 

 3 J. H. Leek and C. S. Martin, Rev. Sci. Instr., 28, 119 (1957). 



