1. instrument for measuring or comparing small electromotive forces, such as those produced by thermocouples.

2. instrument for comparing small electromotive forces, or for measuring small electromotive forces, by comparison with a known electromotive force. Its principal advantage is that during the measurement, no current flows through the source of electromotive force.

3. three-terminal rheostat, or a resistor with one or more adjustable sliding contacts, that functions as an adjustable voltage divider.