It was the forerunner of Kelvin's quadrant electrometer (described below). Kelvin used similar photographic means for his quadrant electrometer (see above) in the 1860s. This generates an ionization current which is measured by an electrometer circuit. To aid this work they invented the piezoelectric quartz electrometer. A more sensitive form of quadrant electrometer was developed by Frederick Lindemann. The collector current is measured in picoamperes by an electrometer. This current is amplified and displayed by a high-gain differential amplifier/electrometer. As a recording device he first used the Lippmann's capillary electrometer, but results were disappointing. An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. A "log R" amplifier or electrometer is used to measure the resistance.