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.


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