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Siemen’s Electrodynamometer

Siemen’s electrodynamometer is an instrument used to measure currents in terms of deflection of two perpendicular coils suspended by a spring. It consists of two coils, one fixed (F1F2) and another movable (N1N2). The movable coil is suspended by a torsion fiber.

One end of the spring S is connected to the movable coils and the other is connected to the torsion head T. The twist of the coil is controlled by this spring. The torsion head T carries a pointer P1. By working the torsion head, the coil may be brought back to the original zero position. P1 moves over a circular scale. A pointer P2 is attached to the moving coil. This serves as reference point. M1 and M2 are mercury cups one above the other into which the lower ends of the coils dip.

The two coils are connected in series. When no current passes, the movable coil is arranged so that its plane is perpendicular to the plane of the fixed coil. The plane of moving coils arranged perpendicular to magnetic meridian so that the earth’s magnetic field has no effect on its deflection. Now P2 indicates zero. The current to be measured is passed through the two coils in series. The forces developed tend to set the moving coil parallel to fixed one. The torsion head with pointer P1 is then turned in opposite direction to the deflection until the pointer P2 is again at zero. Let i be the current and θ the deflection through which P1 and turned

Magnetic induction field produced by the fixed coil, B ∞ i.
Torque on coil (N1N2) = magnetic moment x B
            ∞ i x i
            ∞ i2
The deflection is θ in the torsion wire.
The restoring torque = Cθ and it is equal to deflection is θ in the torsion τ.
    . :        Cθ ∞ i2
            θ ∞ i2
    . :        i = k√θ
Since θ ∞ i2, AC and DC can be measured by this instrument.

Siemen Electrodynamometer

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