248 HOWARD J. CURTIS 



C. EQUIPMENT EOR POTENTIAL MEASUREMENTS 



The problem of equipment for measuring these potentials is as old 

 as the science of electrobiology, and still continues to be a pi'oblem. 

 Each time there has been an impi-ovement in electrical measuring 

 equipment in phj^sics or engineering, someone has been quick to ap- 

 ply it to problems of potential measurements in biology. In some 

 cases new types of electrical measuring equipment have been devised 

 by biologists to meet the stringent demands of particular biological 

 problems. For example, Einthoven developed the string galvanom- 

 eter solely for the purpose of measuring the electrical potential 

 changes associated with the heart beat. 



Prior to the development of the radio tube, a number of instru- 

 ments were used for the measurement of biological potentials. 

 Among these can be mentioned the quadrant electrometer, the string 

 galvanometer, the moving-coil galvanometer, and others. All have 

 been important in the development of electrophysiology, but only 

 the moving-coil galvanometer retains any degree of importance. 

 For a general treatment of these older instruments, the reader is re- 

 ferred to Bayliss (3) . 



In general, it is now customary first to amplify the potential being 

 measured by means of an electronic amplifier and then apply the 

 amplified potential to some electrical measuring instrument. The 

 apparatus can then be discussed in two parts. 



1. Electronic Amplifiers 



A complete discussion of electronic tubes is far beyond the scope 

 of this chapter. However, these tubes are extremely useful devices 

 in biological research, and the biologist can use them intelligently 

 without knowing the complete theory behind them in much the 

 same way that he uses a microscope without knowing all of the 

 theory of optics that went into the construction of the instrument. 

 The biologist is advised against taking a course in communications 

 engineering since the material presented there does not fulfill his 

 needs. Instead, he is referred to a book by Muller, Garman, and 

 Droz (20), which contains the elements of experimental electronics 

 as they are needed by the biologist. 



With a relatively elementary knowledge of electronic tubes and 

 circuits it is possible for the biologist to construct and use this equip- 

 ment from published circuit diagrams. As he becomes more profi- 



