A fluxmeter is a galvanometer with a mobile coil of the Deprez D'Arsonval type, in which the coil is mounted without a return spring on a vertical pin the points of which lie, with no perceptible friction, on their supports; the coil receives the current from across two very easily bent disks of annealed silver.
In this galvanometer, as in those for ballistics, under the action of an impulse of current the deviation of the needle is proportional to the total charge that the impulse caused to circulate and the proportionality constant is given by the product of total resistance (of the galvanometer and the circuit in which it is included) by a constant that depends only on the characteristics of the instrument.
If a coil is attached to the galvanometer, a rapid variation in induction flux linked to this generates an impulse of induced current and, by the law of Faraday-Neumann, the charge that circulates is equal to the variation in flux divided by total resistance.
All things considered, therefore, the deviation of the galvanometer needle is proportional to the variation in flux of induction connected to the coil that caused it and the proportionality constant depends only on the characteristics of the instrument. The constant being known, the instrument allows measurement of the variation in flux and it is for this reason that it was given the name of fluxmeter.
To measure flux, it is sufficient to place the coil so that it is normal to the direction of the magnetic field and then turn it quickly by 90 degrees so that the flux goes from the maximum value to zero, or by 180 degrees so that the maximum value goes to the opposite value. If the number and area of the loops on the coil are known, it is possible to measure the mean value of the vector of magnetic induction in the region originally occupied by the coil. To measure the magnetic induction vector between the pole pieces of a magnet, it is sufficient to place the coil between them and then remove it quickly and place it far from the magnet, where the magnetic field is null.
The fluxmeter presented here is protected by a robust aluminium alloy case painted black, the base of which lies on three adjustable feet.
On the cover, besides a small round spirit level, there is a window for the reading of the deviations on the scale with zero at the centre, graduated from 0 to 75, and a mirror to avoid the parallax error. The instrument is endowed with two square coils (mean side 6.4 cm), one with 50 and the other with 200 loops. They are attached at a vertex to one of the two extremities of a bakelite plate (9 cm x 2.5 cm) which also acts as a handle, while at the other extremity there are two connecting terminals for connection to the galvanometer.

Perucca (1949), p. 772

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