An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern electrometers based on vacuum tube or solid-state technology can be used to make voltage and charge measurements with very low leakage currents, down to 1 femtoampere. A simpler but related instrument, the electroscope, works on similar principles but only indicates the relative magnitudes of voltages or charges. Historical electrometers Gold-leaf electroscope The gold-leaf electroscope was one of the instruments used to indicate electric charge. It is still used for science demonstrations but has been superseded in most applications by electronic measuring instruments. The instrument consists of two thin leaves of gold foil suspended from an electrode. When the electrode is charged by induction or by contact, the leaves acquire similar electric charges and repel each other due to the Coulomb force. Their separation is a direct indication of the net charge stored on them. On the glass opposite the leaves, pieces of tin foil may be pasted, so that when the leaves diverge fully they may discharge into the ground. The leaves may be enclosed in a glass envelope to protect them from drafts, and the envelope may be evacuated to minimize charge leakage. A further cause of charge leakage is ionizing radiation, so to prevent this, the electrometer must be surrounded by lead shielding. This principle has been used to detect ionizing radiation, as seen in the quartz fibre electrometer and Kearny fallout meter. This type of electroscope usually acts as an indicator and not a measuring device, although it can be calibrated. A calibrated electrometer with a more robust aluminium indicator was invented by Ferdinand Braun and first described in 1887. According to Braun, the standard gold-leaf electrometer is good up to about with a resolution of using an ocular micrometer. For larger voltages up to Braun's instrument can achieve a resolution of . The instrument was developed in the 18th century by several researchers, among them Abraham Bennet (1787) and Alessandro Volta. Early quadrant electrometer While the term "quadrant electrometer" eventually referred to Kelvin's version, this term was first used to describe a simpler device. It consists of an upright stem of wood, to which is affixed to a semicircle of ivory. From the center there hangs a light cork ball upon a pivot. When the instrument