The usual thermionic valve circuits for amplifying the small measuring current in potentiometric methods cause serious errors due to the appreciable grid current present in valves having high mutual conductance. On the other hand, the use of electrometer valve is attended with lack of sensitiveness. A d.c. amplifier has been described using an electrometer triode coupled to a valve of LP2 type. The drift was considerably less than in a single valve circuit and electrode potentials could be measured to 0·1 millivolt using a unipivot galvanometer. A simple high-insulation switch is designed which enables both the standard and secondary cells to be operated with ease.