An instrument for the measurement of the electric potential difference between two conductors. Many different kinds of instruments are available to suit different purposes. Voltages of the order of picovolts (10⁻¹² V) to megavolts (10⁶ V) can be measured. Frequencies from zero (dc) to many megahertz and accuracies in the range from a fraction of part per million (ppm) to a few percent may be covered.