ELECTRICAL MEASUREMENTS 



185 



be used. Such a circuit employs gas-filled tubes, in which the voltage 

 across the tube is independent of current, for reference. 1 he voltage from 

 the filter is compared continuously with the gas-filled tube, and any 

 tendency to increase or decrease is counteracted. There are several means 

 of achieving this control, some of which become quite elaborate. 



AAA 



(mm 



Fig. 13-7. Rectifier circuits. Top: half-wave, using one diode. Bottom: 

 full-wave, using two diodes (commonly enclosed in the same glass shell). 



In addition to the high voltage for the amplifier tubes, the power 

 supply is likely to provide a set of low voltage a-c or d-c supplies for 

 heaters in vacuum tubes or for other purposes. 



Amplifiers 



The basic unit of the vacuum tube amphfier is the triode. The signal 

 to be amplified is fed into the grid of the tube, where it influences the 

 passage of electrons between the cathode and the anode. Signal fluctua- 

 tions appear on the anode amplified 30 to 100 times. Several stages are 

 frequently used in amplifiers, arranged so that the output of one stage is 

 further amplified by the next stage. Total amplification may be 10,000 

 or more times. 



The easiest amplifiers to build are those for alternating signals. The 

 successive stages are connected through capacitors so that the alternat- 

 ing signal can pass, but any direct current cannot. A direct-coupled 

 amplifier is more useful in biology because it will amplify direct cur- 

 rents as well as alternating currents. It is unfortunate that a very similar 



