Millivoltmeters *

In a previous paper (Burr et al., 1936), a technique was described for the measurement of relatively steady state potential differences in living systems. The technique was designed to measure one aspect of a primary property of protoplasm, the electro-dynamic field. Its purpose was to provide a procedure which would give reproducible and predictable measurements of relatively pure voltage differences with a minimum disturbance to the system being measured. The input impedance of the circuit was set at 10 megohms as a practical compromise between a current drain instrument and an instrument with such a high input impedance as to be sensitive to a great variety of electrical disturbances. The instrument was designed to be effective at the microvolt range, to be sufficiently stable, and to be independent of the ordinary changes in the electrical environment. The original circuit employed 112-A tubes and silver-silver chloride electrodes. These electrodes were prepared by chloriding silver oxide. In the years intervening since the publication of the original technique, constant search has been made for improved and simpler procedures. Recently this has been made imperative by the cessation of manufacture of the 112-A's. However, practice has made it clear that no basic changes in the original theory of the instrument were necessary. Minor modifications were necessary only as a result of the appearance of more modern vacuum tubes. Moreover, it has become obvious that the great majority of measurements in laboratory animals and in humans are in the millivolt, not the microvolt range. This has reduced some of the earlier rigorous requirements. High insulation of switches is no longer required, nor is it necessary to remove tube bases. Insulating the equipment and batteries on glass can also be dispensed with. The lesser sensitivity has made unnecessary the rather elaborate technique in the preparation of the electrodes. Of the large number of changes which have been tried, four variants have proved effective. The electronic bridge circuit satisfies two criteria for a good DC millivoltmeter, namely, simplicity and stability.