250 HOWARD J. CURTIS 



tracted to the grid provided it is negative. Thus the grid current 

 will be positive or negative depending on the magnitude and sign of 

 the grid potential. It then follows that at some grid potential the 

 grid current will be zero and the grid resistance (input resistance) will 

 be infinite at this potential. This cannot be fully achieved, but some 

 very close approximations can be made. 



Actually, of course, there are many causes of grid current, many 

 of which can be minimized by careful tube design and construction. 

 Such tubes are manufactured, and are known as electrometer tubes. 

 In such tubes all other characteristics are sacrificed in favor of a low 

 grid current. 



A large number of circuits have been published for use with these 

 electrometer tubes, each desirable for a particular application. How- 

 ever, there has been a large demand for such circuits to be used in 

 connection with glass electrodes. A number of commercial com- 

 panies now manufacture these glass electrode amplifiers, and they 

 function very satisfactorily. It would therefore be inadvisable for a 

 biologist to attempt to build one of these amplifiers, since their de- 

 sign and construction are quite critical. The commercial glass elec- 

 trode amplifier can be adapted readily to almost any use requiring a 

 very high resistance input. For special applications it may be neces- 

 sary to construct such an amplifier, and for this the reader is referred 

 to articles by Turner (22), DuBridge and Brown (13). Penick (21), 

 and Bearden (4) . 



3. Direct Current Amplifiers 



It is seldom necessary, in biological work, to resort to the use of 

 electrometer type amplifiers, and a general purpose amplifier will 

 usually suffice. With almost no precautions such amplifiers draw 

 an input current of 10"'^ amp. or less and with reasonable precautions 

 will go down to 10 ~^ amp. or less. If it is necessary to go below 10 "^"^ 

 amp., an electrometer circuit should be employed. 



A curve of grid current versus grid voltage is shown for a typical 

 triode in Figure 6. It will be seen that if the grid voltage is always 

 less than — 1 v., the grid current will never be more than about 10 ~^ 

 for this particular tube. However, if the grid voltage becomes more 

 positive than about —0.5 v., the grid current may be relatively large. 

 This curve shows the fallacy of the statement one often hears that 

 the grid current will he negligible as long as the grid voltage is nega- 

 tive. From the curve of Figure 6 it will be seen that at about —0.75 



