Historical electrometers
– Gold-leaf electroscope
– Used to indicate electric charge
– Consists of two thin leaves of gold foil
– Leaves acquire similar electric charges and repel each other
– Glass envelope protects leaves from drafts
– Can be enclosed in lead shielding to prevent charge leakage
– Early quadrant electrometer
– Consists of stem, semicircle of ivory, and cork ball
– Stem participates and repels the cork ball when placed on a charged body
– Measured angle is not in direct proportion to the charge
– Invented by William Henley and Horace-Bénédict de Saussure
– Coulomb electrometer
– Uses torsion to measure charge
– Glass cylinder with a glass thread and gum lac rods with gilt balls
– Repulsion of movable balls indicates charge
– Improved by Francis Ronalds with a suspended needle and Faraday cage
– Forerunner of Kelvin’s quadrant electrometer
– Peltier electrometer
– Developed by Peltier
– Uses a magnetic compass to measure deflection
– Balances electrostatic force with magnetic needle
– Measures charge accurately
– Alternative to other electrometers
Modern electrometers
– Highly sensitive electronic voltmeter
– Input impedance is so high that the current flowing into it can be considered zero
– Input resistance for modern electronic electrometers is around 10 Ω
– Special design considerations, such as driven shields and special insulation materials, are required to avoid leakage current
– Commonly used in nuclear physics experiments and for measuring radiation with ionization chambers
– Vibrating reed electrometers
– Use a variable capacitor formed between a moving electrode and a fixed input electrode
– Capacitance varies as the distance between the electrodes changes
– Electric charge is forced in and out of the capacitor, producing an alternating current signal
– DC input resistance is determined by the leakage resistance of the capacitor
– Reed assembly can be attached by a cable for convenience of use
– Valve electrometers
– Use a specialized vacuum tube with high gain and input resistance
– Input current flows into the high impedance grid and is vastly amplified in the anode circuit
– Valves designed for electrometer use have leakage currents as low as a few femtoamperes
– Handling of valves requires gloves to prevent salts on the glass envelope from providing leakage paths
– Inverted triode circuit reverses the roles of anode and grid to minimize input current
– Solid-state electrometers
– Consist of a solid-state amplifier using field-effect transistors
– Can measure voltage, charge, resistance, and current
– Voltage measured through voltage balancing with a high input impedance circuit
– Current measured through a current-to-voltage converter, capable of measuring femtoamperes
– Can measure small electric charges down to a fraction of a picocoulomb
Gold-leaf electroscope
– Used to indicate electric charge
– Consists of two thin leaves of gold foil
– Leaves acquire similar electric charges and repel each other
– Glass envelope protects leaves from drafts
– Can be enclosed in lead shielding to prevent charge leakage
Early quadrant electrometer
– Consists of stem, semicircle of ivory, and cork ball
– Stem participates and repels the cork ball when placed on a charged body
– Measured angle is not in direct proportion to the charge
– Invented by William Henley and Horace-Bénédict de Saussure
Coulomb electrometer
– Uses torsion to measure charge
– Glass cylinder with a glass thread and gum lac rods with gilt balls
– Repulsion of movable balls indicates charge
– Improved by Francis Ronalds with a suspended needle and Faraday cage
– Forerunner of Kelvin’s quadrant electrometer Source: https://en.wikipedia.org/wiki/Electrometer
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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.
