As a gaussmeter measures the magnetic flux density nearly at a point, a flux meter measures the total magnetic flux across an area (that is, the integral of flux density over an area or, for constant flux density, the flux density times the area).

It is more accurate, however, to say that what is measured is not flux, but the change of flux. According to Faraday’s law of induction,

E = N dφ/dt

Where E = voltage across a coil of n turns
φ = iTEGRALB dA = flux (2.60)
B = magnetic flux density
A = area
N = number of turns on the coil linked by flux φ
Integrating this equation,

φ = (1/N) iNTEGRAL (E dt + φ0)

Where φ0 is an arbitrary constant of integration, normally set to zero to begin the measurement.

That is to say, it is possible to measure an amount of flux passing through an object such as a magnet by placing the object to be measured in a tight-fitting coil, then removing the object, while integrating the voltage across the coil with time.

This is the principle of the fluxmeter. Alternately, it is possible to remove the object, rotate it end for end 180°, and reinsert it into the coil. In this case the change of flux is twice that through the object.

The sensor for a fluxmeter is just a coil of wire, usually made at the time by the operator. The wire may usually be of small diameter, because very little current flows during the measurement.

The larger the number of turns, the larger the signal. However, if the coil resistance becomes relatively\ high (possibly 50 Ω or more), some flux meters with relatively low input resistance may require a correction.

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