VIII. BIOELECTRIC MEASUREMENTS 249 



cient in their use he can make changes to fit hi« particular needs, and 

 later will not find it difficult to design his own equipment. With this 

 in mind, several electronic circuits will be reproduced here without 

 elaborate explanation. 



Electronic amplifiers are used in measuring bioelectric potentials 

 for two reasons : first because of their very high input resistance, and 

 second because of their speed of response. When a measuring in- 

 strument or amplifier is connected to the electrodes in an aqueous 

 system, the instrument becomes an integral part of the equivalent 

 circuit and may significantly alter the potential distribution and thus 

 cause considerable error. It will be realized from the above discus- 

 sion of measurement errors that errors can be avoided only if the resist- 

 ance of the measuring instrument is very much higher than that of the 

 circuit being measured. The resistance of many biological circuits 

 can be thousands and even millions of ohms, so in general the input 

 resistance of a satisfactory measuring instrument must be many 

 megohms. Electronic amplifiers and electrometers are the only in- 

 struments satisfying this condition. The latter is extremely difficult 

 to set up with the requisite sensitivity, and even so is a very sluggish 

 instrument, difficult to use and extremely sensitive to moisture. On 

 the other hand, the electronic amphfier can be constructed with al- 

 most any desired input resistance and sensitivity, and can be made 

 to actuate almost any desired recording instrument. 



Amplifiers may be put in three general classes for bioelectric 

 potential measurements: (1) Very high resistance (electrometer 

 type), in which the primary consideration is a high input resistance; 

 (3) general purpose, direct-coupled amplifiers ; (3) capacitor-coupled 

 amplifiers for measuring rapid changes in potential. Each of these 

 will be briefly considered. 



2. Electrometer Amplifiers 



An electronic tube has often been described as a valve in which the 

 flow of electrons between cathode and plate is regulated by the poten- 

 tial on the grid. Accorchng to this view, there would be no flow of 

 current in the grid circuit at all, and indeed this is true to a first ap- 

 proximation. Howevei-, some of the electrons in the tube adhere to 

 the grid, causing some grid cutrent, and this can become quite large 

 if the grid becomes positive wit h i-espect to the cathode. Also, there 

 are always a few positive ions present in the tube due to electron 

 bombardment of the i-osidual gas of the tube, and these will be at- 



