Electrical measuring instruments. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electricity for beginners
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Electrical measuring instruments are designed to measure all kinds of electrical quantities. Conventionally, they can be divided into direct evaluation devices and comparison devices.
In the devices of the first group, the scale is marked in those units that are directly measured by deflecting the arrow. This group includes ammeters, voltmeters, ohmmeters, etc.
In devices of the second group, physical phenomena are used that move the moving system of the device and thereby create a torque. It can be created by the interaction of the magnetic field of a permanent magnet and the magnetic field of a coil, as well as the magnetic field of a coil with current and a ferromagnet, etc.
Depending on what kind of physical process is used in the device, they are divided into devices of magnetoelectric, electromagnetic, electrodynamic, induction, thermoelectric and other systems.
Each measuring device has its own errors. The allowed errors, depending on the properties and quality of the device, determine the accuracy class of this device. The accuracy class, as a rule, is indicated on the scale or in the instrument's passport. There are 8 accuracy classes in total.
The most widespread are devices whose operation is based on an electromagnetic system. This technical device is a fixed coil included in the circuit. Inside the coil there is a core made of soft iron and mounted eccentrically on an axis, on which an index arrow and a coil spring are also fixed.
The spring creates a counteracting moment and returns the arrow to its original position in the absence of current. There is also a piston moving in an air cylinder. The piston plays the role of a damper (air damper).
Devices of the electromagnetic system are designed to measure the strength of direct and alternating current. In devices with an iron core, as a rule, the accuracy class is low. They are used for measurements on panels and for measurements that do not require high accuracy. In laboratories, devices with cores made of an iron-nickel alloy are usually used.
The positive qualities of such devices are the suitability of measurements in both direct and alternating current circuits, resistance to overcurrent, ease of manufacture and good mechanical strength.
The downside of these technical devices is the unevenness of the scale, the occurrence of residual magnetization of the core, as well as the dependence of measurements on external magnetic fields.
Author: Smirnova L.N.
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