VIII. BIOELECTRIC MEASUREMENTS 259 



power supplies and some auxiliary amplifiers. In general these are 

 useful only for measuring potential changes, since the amplifiers are 

 usually capacitance-coupled. It is possible to modify these instru- 

 ments, or build one from parts, in which the plates are coupled di- 

 rectly to the amplifier o-itput so both direct currents and potential 

 changes maj'' be measured. A description of these instruments and 

 hoAV they can be constructed is beyond the scope of this book, and 

 the reader is referred to such books as the one by Muller, Garman, 

 and Droz (30) for further details. 



It is often convenient to record potentials on moving paper, either 

 photographically or by other means. Of these, the photographic 

 methods are by far the most accurate but not the most convenient. 

 Recording oscillographs are available which use a mirror galvanometer 

 that will accurately reproduce an alternating current up to 5000 cycles 

 per second. They give a linear displacement on the moving paper, 

 and may have a total displacement of 12 cm. However, they all have 

 the disadvantages of a photographic method. 



The direct-recording oscillographs do so either by a moving pen 

 (ink writers) or some other marking device such as a heated stylus 

 marking on paraffin paper. These are definitely limited in fre- 

 quency response, usually being able to record frequencies only up to 

 about 100 cycles per second. Also, they record over the arc of a 

 circle and for a width of only about a centimeter. In general they 

 are more satisfactory for qualitative than for quantitative work. 

 However, they are very convenient to use, and this feature offsets 

 their disadvantages for many applications. 



For very slow changes, or for measuring direct currents, an ordi- 

 nary milliammeter is satisfactory for most purposes. A permanent 

 record may be obtained by using a recording milliammeter. 



D. BIOELECTRIC POTENTIALS 



1. Membrane and Action Potentials in Nerve and Muscle 



As indicated in a previous section, an electrical potential exists 

 between the inside and outside of living cells that gives a clue to the 

 function of the cell, and for this reason has been extensively studied 

 in the past. It is known as the membrane potential and can be 

 measured either directly or indirectly. In the direct method an elec- 

 trode is inserted inside the cell and a measurement of the potential is 



