# A model galvanometer

##### Class practical

The deflection of a compass needle increases with the current carried by a wire.

#### Apparatus and materials

Copper wire, PVC-covered, 150 cm with bare ends

Plotting compass

Carbon resistor, 10 ohm

Hardboard piece to take plotting compass

Carbon resistor, 100 ohm

Rheostat (10 - 15 ohms) e.g. from circuit teaching kit

#### Health & Safety and Technical notes

The piece of hardboard should have a central hole to take the plotting compass, and four notches to hold the wire.

The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2-volt terminals. Many 0-12-volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils may become hot.

#### Procedure

a Leaving a length of about 30 cm (for making connections to the power supply), wind five turns of wire on one side of the board. Leave a gap, and wind a further five turns on the other side.

b Insert the compass into the hole in the board.

c Place this device on the bench so that the compass needle is parallel to the winding.

d Connect up to the power supply through a 100Wcarbon resistor. A small current (about 10 mA) will flow; observe the deflection of the compass needle. Reverse the current and repeat.

e Repeat with a 10Wcarbon resistor, and then with a rheostat (so that you can vary the current).

#### Teaching notes

1 Direct connection to the power supply with no resistor should be avoided. The large current sets the needle spinning.

2 For those interested in historic apparatus, this is a model of the tangent galvanometer in which the size of the current passing through the coil is proportional to the tangent of the angle through which the needle moves when there is a current in the coil. The Earth's magnetic field pulls the needle one way, and the coil's field pulls it at right angles.

This experiment was safety-checked in April 2006