Electrometer
[1] 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.
This principle has been used to detect ionizing radiation, as seen in the quartz fibre electrometer and Kearny fallout meter.
A light cork ball hangs by a string from a pivot at the center of the semicircle and makes contact with the stem.
In the axes of the tube is a glass thread, the lower end of this holds a bar of gum lac, with a gilt pith ball at each extremity.
Francis Ronalds, the inaugural Director of the Kew Observatory, made important improvements to the Coulomb torsion balance around 1844 and the modified instrument was sold by London instrument-makers.
Both were metal, as was the suspending line and its surrounding tube, so that the needle and the fixed piece could be charged directly through wire connections.
The original design uses a light aluminum sector suspended inside a drum cut into four segments.
The plates can be connected externally in the conventional diagonal way (as shown), or in a different order for specific applications.
Francis Ronalds created an early electrograph around 1814 in which the changing electricity made a pattern in a rotating resin-coated plate.
Among other applications, electrometers are used in nuclear physics experiments as they are able to measure the tiny charges left in matter by the passage of ionizing radiation.
The most common use for modern electrometers is the measurement of radiation with ionization chambers, in instruments such as geiger counters.
The alternating current signal produced by the flow of this charge is amplified and used as an analogue for the DC voltage applied to the capacitor.
The input current is allowed to flow into the high impedance grid, and the voltage so generated is vastly amplified in the anode (plate) circuit.
Such valves must be handled with gloved hands as the salts left on the glass envelope can provide leakage paths for these tiny currents.
The external connections are usually of a co-axial or tri-axial design, and allow attachment of diodes or ionization chambers for ionising radiation measurement.
A similar circuit modified to act as a current-to-voltage converter enables the instrument to measure currents as small as a few femtoamperes.
