186 ELECTRICAL MEASUREMENTS 



abbreviation, D. C, is used for direct-coupled as for direct current. 

 Direct-coupled amplifiers come with built-in problems, and their design 

 is best left to the electronics engineer. The problems are so serious that 

 once they are solved the result is a very fine amplifier indeed. 



Biological amplifiers have some special requirements. The "hi-fi" 

 amplifier is constructed so that it provides the same amplification re- 

 gardless of frequency over a wide range. The "addict" is proud to state 

 that his amplifier is "flat" from 5 to 50,000 cycles per second. Biological 

 signals are direct current, or very low frequency alternations, so this 

 fine frequency response is wasted. If a response like a nerve potential 

 is to be measured, then somehow the nerve cell must be connected to the 

 grid of the first amplifier stage. A small voltage (bias) must be applied 

 to the grid, however, and this bias is likely to have "unbiological" con- 

 sequences in the nerve cell. Or, to put somewhat the same idea in a 

 different way, if a pair of electrodes is attached to a cell or tissue, and the 

 impedance through the external circuit is lower than the impedance of 

 the cells, then any electrical potential in the cells results in a current in 

 the external circuit instead of in the cells where it belongs. Therefore, 

 some form of high impedance input circuit must be used with biological 

 systems in order to prevent the instrument from influencing the cells. 



Transistor amplifiers offer several advantages in biological experimen- 

 tation by partially overcoming some complex problems. One of the dif- 

 ficulties in the use of a conventional amplifier for measuring biological 

 reactions is the set of electrical properties at the junctions between the 

 electrodes and the cells. Transistor amplifiers permit the use of electrode- 

 tissue junctions with less tendency to "damp" or obscure the biological 

 signal. Another advantage is that transistor amplifiers give their best 

 performance in the range of frequencies encountered in biological ex- 

 periments. Finally, the small size and low power requirements permit 

 the use of transistor amplifiers in situations where a vacuum-tube am- 

 plifier would be impractical. Transistor engineering is very complex, 

 so even the biologist who is quite competent in electronics does not try 

 to design his own circuits. 



Potentiometric techniques 



A number of laboratory instruments measure electrical potentials (or 

 changes in potentials) which result from chemical reactions. Consider, 

 for example, a piece of copper wire with one end in a solution of a coj> 



