D'Arsonval-Deprez GalvanometerThe two French inventors of this form of galvanometer in the early 1880s came from quite different backgrounds. Jacques D'Arsonval (1851-1940) was a director of a laboratory of biological physics and a professor of experimental medicine, and one of the founders of diathermy treatments. Marcel Deprez (1843-1918) was an engineer and an early promoter of high-voltage electrical power transmission.In the D'Arsonval-Deprez design the coil has many turns of fine wire, and is suspended by flat ribbon of wire which serves as one lead-in wire. The connection to the lower end of the coil is provided by a light, helical spring that provides the restoring torque. The electro-magnetic torque is greatest when the magnetic field lines are perpendicular to the plane of the coil; this condition is met for a wide range of coil positions by placing the cylindrical core of soft iron in the middle of the magnetic gap, and giving the magnet pole faces a concave contour. The electro-magnetic torque is proportional to the current in the coil and current flowing through the coil is linearly proportional to its angular deflection. This means that the galvanometer scales can always be linear, a great boon to the user.This model of the metering device gradually became the main one and replaced the tangential and sinusoidal galvanometers which had poorer accuracy and were to a bigger extent dependent on the outer magnetic fields. This device is true ancestor of today's magneto-electrical type devices.

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