THERMIONIC VACUUM TUBES 



99 



of the tube, 7+ the positive ion current, and 7— the electron current. 

 As it is necessary to know the value of I—, and since the emission 

 from the filament is liable to vary somewhat with the kind of gas and 

 its pressure, it will be found advantageous, if many readings are to be 

 made, to place in the 7— circuit, the coils of a relay which is adjusted 

 to close at a definite value of 7—, and which, when closed, cuts in a 

 shunt around the filament which will reduce its heating current. 



fc-W* 



Fig. 79 

 Automatic regulators of this type have been used with complete suc- 

 cess. Experiment shows that the value of K remains constant for 

 pressures as high as 1.5 X10 -3 mm. of Hg. and the lower limit is de- 

 termined very largely by the sensitivity of the current reading in- 

 struments. It follows that an ionization gauge can be calibrated by 

 comparison with a McLeod gauge and, in use, extrapolated to low 

 pressures. 49 



60. Heterodyne Method of Generating Currents of Very Low Fre- 

 quencies. By impressing upon the grid of a detector tube two fre- 

 quencies which differ by a very small amount (e.g., 99 and 100 cycles), 



49 Dushman, Phys. Rev., Oct., 1920, p. 854. 



