Some Examples of the Effect of Self-induced Current

A resistance box contains several wire coils of different resistances. These coils are wound using a doubled wire on wooden cylinders.

Why are the coils doubly wound?

• At every point in the coil, current flows in two opposite directions.
• Therefore, the magnetic flux linked with the coil remains almost zero.
• This arrangement is called non-inductive winding.
• Hence, the effect of self-inductance in the coil becomes negligible.

Important: Non-inductive winding makes the effect of self-inductance negligible in a resistance-box coil.

Correct Procedure

1. Press the cell key first.
2. Then press the galvanometer key.
3. After taking the reading, release the galvanometer key first.
4. Then release the cell key.

Reason

This precaution avoids the effect of self-inductance in the coils of the Wheatstone bridge.

• If the galvanometer key is pressed first and the cell key is pressed afterwards, momentary currents are induced in the coils of the different arms of the bridge.
• Due to these induced currents, the main current takes some time to reach its steady value.
• Even if the bridge is exactly balanced, the galvanometer may show a momentary deflection.
• This can create the false impression that the bridge is not balanced.

Remember: Press the cell key first, then the galvanometer key. Release the galvanometer key first, then the cell key.

• Resistance-box coils are wound with doubled wire.
• Current in adjacent portions flows in opposite directions.
• Magnetic flux linkage remains almost zero.
• This is called non-inductive winding.
• It makes the effect of self-inductance negligible.
• In a Wheatstone bridge, press the cell key before the galvanometer key.
• This prevents confusion caused by momentary galvanometer deflection.
• After taking the reading, release the galvanometer key before the cell key.