The current balance

Demonstration

The current balance is a useful meter. Sophisticated versions of it are used in very sensitive current-measuring experiments. However, it is not very portable or easy to use.

Apparatus and materials

Sellotape

Lamps with holders, 3

Copper wire, bare, 26 SWG, 1 reel

Cells, 1.5 V, with holders, 2

Current balance

Health & Safety and Technical notes

Circuit boards with 2 cells and 3 lamps may be used.

If the straw gives trouble by slewing round, cut a shallow groove accurately in the vertical rails with a file to localize the straw.

Beware of draughts. Warn students not to wait for the straw to come to rest.

The magnet/straw assemblies are quite fragile, and care should be taken to preserve them between lessons.

Procedure

a Connect the balance into a circuit of one lamp and one cell in order to weigh 'one lamp's worth' of current.

b Reverse the connections to the balance and see what happens.

c With one lamp in the circuit, move the rider along the drinking straw until it is again horizontal with the end against the reference mark. This position of the rider will represent the current and can be marked.

d Put an extra lamp in series so that the lamps are under-run and only glow faintly. Move the rider to balance the straw again.

e Increase the voltage so that one lamp glows more brightly and the current is greater. Again move the rider to balance the straw.

Teaching notes

1 The magnetic effect of an electric current can be used to indicate the size of an electric current. When a current passes through a coil of wire, the coil of wire behaves like a magnet. The magnet attached to the straw is then attracted towards the coil, and the straw becomes tilted. The force between the two magnets can then be balanced by putting a counterweight on the other end of the straw.

The greater the current, the greater the force between the magnets. The counterweight must be positioned further away from the fulcrum to return the straw to the horizontal position.

2 The current balance is useful in showing what happens when its leads are connected the opposite way round. The straw moves down and cannot be brought into balance with the rider. A useful lesson is that instruments may have to be connected in a particular way and that is why they frequently have red and black terminals. Of course the current balance does not indicate the current direction.

This experiment was safety-checked in March 2005