A direct reading, attracted-disk, absolute voltmeter

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Abstract
An attracted-disk voltmeter is described, which measures voltages up to 30 kv to an accuracy within 0.25%. The voltage to be measured is applied between two parallel plane electrodes, one of which is fixed while the other is suspended from one end of a mechanical balance arm pivoted at its centre. A coil is attached to the other arm of the balance and the force of attraction between this and a permanent magnet is used to provide the turning moment required to counteract that due to the voltage. The balance condition for any value of applied voltage is established automatically by means of an error-controlled closed-loop electro-optical circuit. A fixed relationship exists between coil current and applied voltage. Thus, the applied voltage can be read directly. The instrument is calibrated absolutely by balancing known weights against the attractive force between the disks produced by given voltages.

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