vm. 



B I O E L E C T J{ I C MEASUREMENTS 



251 



V. the grid current is zero. Thus, if provision is made for running 

 the tube always exactly at this voltage, the grid current can be kept 

 extremely low. This principle forms the basis of several instruments 

 designed for biological work in which it is desirable to keep the grid 

 current low. However, these circuits are somewhat awkward to use 

 and are not recommended. 



It should be emphasized that the curve of Figure 6 is for one par- 

 ticular triode, operating at one particular plate voltage. Other 

 tubes show curves of the same form but the magnitude of current and 

 voltages may be quite different. If there is any question as to how 



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Fig. 6. Grid current vs. grid voltage for a typical triode. 



low the grid current is in any particular amplifier, it should be meas- 

 ured directly. This can be done conveniently by placing a high re- 

 sistance (100 megohms or more) of known value in series with the grid 

 and noting the plate current. Then the resistance is replaced by a 

 low resistance potentiometer and a potential applied that will give 

 the same plate current. The grid potential must have been the same 

 in both cases, which means that the potential drop across the resistor 

 is known. From this, and knowing the value of the resistor, the 

 grid current can be computed from Ohm's law. If a very low grid 

 current is necessary, it is desirable to plot out a grid current versus 

 grid voltage curve for the particular tube being used and the par- 

 ticular way in which it is used. 



One of the most troublesome features of direct current amplifiers 

 is drift. It is practically impossible to eliminate drift completely, 

 but a few simple precautions will enormously reduce it. The first 

 thing to do is to use a balanced amplifier throughout, so the drift of 



