A Precision Galvanometric Instrument for Measuring Thermoelectric E. M. Fs

A new principle has been developed whereby an ordinary millivoltmeter may be converted into an instrument in which the usual errors arising from a variable line resistance are entirely eliminated. The instrument measures true e. m. f. in a simple circuit or if connected across a resistance or network through which a current flows it indicates the potential drop which would have existed had the instrument not been connected. In this respect it functions as a potentiometer, yet it does not operate on the potentiometric principle since it does not require a standard cell (or the equivalent) or an auxiliary battery, the only e. m. f. employed in the adjustment being that of the source measured. No loss in precision of setting results; in fact the adjustment may be readily made to 10 times the scale accuracy. Various wiring diagrams are shown and methods are discussed for constructing instruments of zero temperature coefficient and properly damped. A new deflection potentiometer is described which offers (considerable advantage over the ordinary type for small e. m. fs. in a circuit of variable resistance.