The following experiments originated in a series of laboratory exercises which I performed on the use of the Dolezalek electrometer. The instrument which I used was made by Bartels, and had its needle suspended by a phosphor-bronze strip. This needle was raised to a potential of about 160 volts by being connected to one terminal of a battery of eighty small secondary cells, the other terminal of which was connected to earth. When at this potential the instrument gave, for a potential difference of 1 volt between its two pairs of quadrants, a deflection of 200 divisions on a scale 200 centimetres distant from it. The maximum deflection in the experiments described was 166 divisions, i.e. 33·2 cms., giving tan−1 ·166 as maximum angle of deflection; and therefore, by taking the deflection as proportional to the potential difference, a maximum error of 0·7 per cent, is introduced.