THERMIONIC VACUUM TUBES 



95 



X. Miscellaneous Applications of Thermionic Vacuum Tubes 



56. The Tube as a Voltmeter. The three-element tube may be 

 used for the measurement of either d.c or a.c. voltages. In the case 

 of d.c. voltages it is customary to apply the unknown voltage to the 

 plate, counter-balancing this voltage with a known negative potential 

 applied to the grid. Given the n of the tube, it is then possible to 

 calculate with a fair degree of accuracy the plate potential. The 

 usual procedure is to adjust the negative grid potential to such a 

 point that the plate current just becomes zero. The tube when used 

 in this manner becomes an electrostatic voltmeter, and it is evident 

 that to give accurate readings the tube should have a well-defined 

 cutoff (see Sec. 8, Fig. 10). 



In a somewhat similar fashion a.c. peak voltages may readily be 

 compared with known d.c. voltages. A typical circuit is shown in 

 Fig. 69. In operation a fixed plate voltage is applied to the voltmeter 



Fig. 69 



Fig. 70 



tube and a steady negative d.c. voltage is applied to the grid which is 

 just sufficient to reduce the plate current to zero. The a.c. voltage is 

 then superimposed in the grid circuit with the result that current 

 flows during the positive halves of the wave. If now the steady 

 grid potential is made more negative until the plate current again 

 just ceases to flow, it is apparent that this change in the steady poten- 

 tial just equals the peak value of the a.c. voltage. 



For the measurement of very high voltages a special tube of the 

 design shown in Fig. 70 will be found desirable, the grid being in the 

 form of a screen which surrounds the filament. Such a tube may have 

 a fi as high as 200. 



A circuit similar to Fig. 69 may be so employed that the a.c. voltage 

 to be measured causes a change in the space current meter reading, 

 the negative grid potential being preferably so set that the conditions 

 discussed in Sec. 16 are satisfied. The tube, due to its curved char- 



