for modern electronic electrometers is around 1014Ω, compared to around 1010Ω for nanovoltmeters. Owing to the extremely high input impedance, special design considerations (such as driven shields and special insulation materials) must be applied to avoid leakage current. 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. Vibrating reed electrometers Vibrating reed electrometers use a variable capacitor formed between a moving electrode (in the form of a vibrating reed) and a fixed input electrode. As the distance between the two electrodes varies, the capacitance also varies and electric charge is forced in and out of the capacitor. 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 DC input resistance of the electrometer is determined solely by the leakage resistance of the capacitor, and is typically extremely high, (although its AC input impedance is lower). For convenience of use, the vibrating reed assembly is often attached by a cable to the rest of the electrometer. This allows for a relatively small unit to be located near the charge to be measured while the much larger reed-driver and amplifier unit can be located wherever it is convenient for the operator. Valve electrometers Valve electrometers use a specialized vacuum tube (thermionic valve) with a very high gain (transconductance) and input resistance. 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. Valves designed for electrometer use have leakage currents as low as a few femtoamperes (10−15 amperes). Such valves must be handled with gloved hands as the salts left on the glass envelope can provide leakage paths for these tiny currents. In a specialized circuit called inverted triode, the roles of anode and grid are reversed. This places the control electrode at a maximum distance from the space-charge region surrounding the filament, minimizing the number of electrons collected by the control electrode, and thus minimizing the input current. Solid-state electrometers The most modern electrometers consist of a solid state amplifier using one or more field-effect transistors, connections for external measurement