The spark micrometer, also known as a Riess micrometer was a device used by 19th century physicists to measure potential in an electric circuit. It was developed principally by German physicist Peter Riess. It consisted of two electrodes very close together, one of which was attached to a micrometer screw with a calibrated dial, so by turning a knob the width of the gap could be adjusted very precisely. From Paschen's law, the distance between two electrodes when a spark just jumped across a gap was proportional to the potential difference (voltage) between the electrodes, so a spark micrometer could serve as a crude voltage measuring instrument, by widening the gap until the voltage was just able to jump across.