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a device that makes possible direct reading of values of a quantity being measured.
In analog instruments the readout is from a scale; in digital instruments, from a digital reading device. Indicating instruments are intended only for visual readings, whereas recording instruments are equipped with a device for recording the readings, mainly on paper. Recording instruments are subdivided into automatic types, which produce a record of readings in the form of a diagram, and printing types, which provide readings in digital form. In direct-action instruments (such as a manometer or ammeter), one or more conversions of the measured quantity are performed and its value is found without comparison with a known like quantity. In comparison instruments the quantity being measured is compared directly with a like quantity reproduced by a standard (examples are an equal-arm balance, an electric potentiometer, and a comparator for linear measures). Integrating instruments (electric or gas meters), in which the quantity is integrated in time or with respect to another independent variable, and summation instruments, which provide the value of two or more quantities fed through different channels (wattmeters, which total the power of several electrical generators), are among the varieties of measuring instruments.
To automate the control of industrial processes, measuring instruments are often equipped with additional recording, calculating, and control devices that function according to predetermined programs.
K. P. SHIROKOV