devices, and usually a display and/or data-logging connections. The amplifier amplifies small currents so that they are more easily measured. 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. The display or data-logging connections allow the user to see the data or record it for later analysis. Electrometers designed for use with ionization chambers may include a high-voltage power supply, which is used to bias the ionization chamber. Solid-state electrometers are often multipurpose devices that can measure voltage, charge, resistance and current. They measure voltage by means of "voltage balancing", in which the input voltage is compared with an internal reference voltage source using an electronic circuit with a very high input impedance (of the order of 1014 Ω). A similar circuit modified to act as a current-to-voltage converter enables the instrument to measure currents as small as a few femtoamperes. Combined with an internal voltage source, the current measuring mode can be adapted to measure very high resistances, of the order of 1017 Ω. Finally, by calculation from the known capacitance of the electrometer's input terminal, the instrument can measure very small electric charges, down to a small fraction of a picocoulomb. See also Electrical measurements Electron Electroscope Radiation Faraday cup electrometer References Dr. J. Frick, Physical Technics; Or Practical Instructions for Making Experiments in Physics Translated By John D. Easter, Ph.D. - J. B. Lippincott & Co., Philadelphia 1862 Robert Mfurgeson Ph.D. Electricity - William and Robert Chambers, London and Edinburgh 1866 Silvanus P. Thompson, Elementary Lessons in electricity and Magnetism. - Macmillan and Co. Limited, London 1905 Jones, R. V., Instruments and Experiences - John Wiley and Sons, London 1988 External links Build this FET electrometer - A very simple circuit - 2 components Simple FET electrometer - A simple bridged circuit An op-amp electrometer Early electrometers Charging an Electroscope by Induction Using a Negatively-Charged Balloon from Multimedia Physics Studios Voltmeters Electrical meters Measuring instruments Electricity