Hibbert Magnetic Standard

It consists of a block of hard steel with a cylindrical slot cut in it. The block is magnetized so that the field is radial. A brass tube has a single layer of coil of n turns wound upon it with its ends connected to the terminals T₁ and T₂. The brass tube which can freely slide into the slot is always dropped into slot from a certain fixed height. The magnetic flux between the north and south poles of the magnet is determined using a solenoid inductor.

The two terminals T₁ and T₂ are conducted to a B.G., through a resistance box R and commentator. A resistance R₁ is introduced in the resistance box. The brass tube is now dropped into the slot between the north and south poles. The change in magnetic flux linked with the coil is Φn where Φ is the magnetic flux in the Hibbert’s standard.

Let G = resistance of the B.G., and the coil.
Induced charge in the coil}    =    Φn / (R₁ +G)

Let θ1 be the corrected first throw in the B.G., due to the flow of this charge. Then
                =    Φn / (R₁ +G) = Kθ₁                 … (1)

Now, another resistance R₂ is included in the resistance box and the experiment is repeated. The mean corrected throw θ1 is found. Then,
        Φn / (R₂ +G) = Kθ₂                             … (2)

From (1) and (2), K = Φn / (R₂ - R₁) (1/θ₁ – 1/ θ₂)                        … (3)