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an instrument used to measure magnetic flux. The most widely used fluxmeters are permanent-magnet and photoelectric fluxmeters.
The permanent-magnet fluxmeter is a permanent-magnet instrument in which the moving element—a light frame suspended by a torsionless suspension—is in equilibrium in any position, since the restoring torque is very small. A deflection of the moving element is proportional to a change ΔΦ in the flux linkage between a search coil, connected to the fluxmeter’s terminals, and the magnetic flux to be measured: ΔΦ = (C/W)(α2 – α1), where W is the number of turns in the search coil, C is the motor constant of the fluxmeter in webers per scale division (Wb/div), and α1 and α2 are the initial and final positions of the instrument’s pointer in scale divisions.
The flux linkage changes when the magnetic field of, for example, a solenoid or an electromagnet is energized or de-energized or when the position of the search coil in the magnetic field is changed. In contrast to the case of a ballistic galvanometer, the readings of the fluxmeter, within certain limits, are independent of the time required for the change in the magnetic flux, which may be as long as several seconds, and are independent of the resistance of the external circuit. Thus, for an external circuit resistance of up to 8 ohms, the types M 19 and M 119 fluxmeters, which are the most widely used types in the USSR, retain their precision class.
The photoelectric fluxmeter is a permanent-magnet galvanometer with a mirror on a moving frame to which a search coil is connected. A spot of light, which is reflected from the mirror, illuminates two identical photocells connected in opposition. When the frame is motionless, the photocell currents are balanced. When the frame turns as a result of the generation of an electromotive force in the search coil, the balance is disrupted, and the resulting voltage, which is associated with the imbalance in the electric circuit, is fed to the input of an amplifier. In the amplifier, the voltage associated with the imbalance is balanced by a feedback voltage, which is proportional to the current in an indicator, for example, a null indicator. The change ΔI in current observed in the indicator is proportional to the change in the flux linkage: ΔΦ = (C/W)ΔI.
Balanced photoelectric fluxmeters have a broader frequency range and a higher sensitivity than permanent-magnet fluxmeters. For example, the motor constant of the type F-190 micro-fluxmeter is C = 4 × 10–8 Wb/div. The output of the type F-190 instrument is connected to a graphic recorder; the instrument can detect and record low-frequency variable magnetic fluxes.
REFERENCESMagnitnye izmereniia. Moscow, 1969.
Kifer, I. I. Ispytaniia ferromagnitnykh maleriatov, 3rd ed. Moscow, 1969.
Chechurina, E. N. Pribory dlia izmereniia magnitnykh velichin. Moscow, 1969. (Elehroizmerit. pribory, issue 13.)
I. I. KIFER