Practical activities designed for use in the classroom with 11- to 19-year-olds.
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Class practical

Switching a small current through one circuit causes a larger current through another circuit to be switched on (or off).

Apparatus and materials

For each student group

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

C-core, laminated iron

Support blocks or clamps, 2

Cell, 1.5 V

Switch

Battery, 12 V or low-voltage DC power supply

Lamp (12 V 24 W) in lamp holder

Health & Safety and Technical notes

Although hacksaw blades are traditionally used for this activity, some schools may consider it necessary to use strips of hard steel without teeth.

The blades must be demagnetised before each lesson because they could display an assortment of magnetic poles along their lengths.

Procedure

a Construct the first circuit as follows:

• Take one iron C-core.
• Wind twenty turns of PVC-covered copper wire round one arm.
• Connect one end to one terminal of the 1.5 V cell.
• Connect the other end to the switch.
• Complete the circuit by connecting the other end of the switch to the free terminal of the cell.

b Clamp one end of the hacksaw blade.

c Tape a length of insulated wire to the blade. Its free, uninsulated, end must project a few centimetres beyond the end of the blade.

d Position the C-core under the projecting blade, but not quite touching it.

e Now construct the second circuit as follows:

• Connect the end of the insulated wire to one end of the 12 V DC battery or supply.
• Connect the other terminal of the supply to the lamp.
• Connect a length of wire to the other terminal of the lamp.
• Tape the bare end of this wire to the top of the second wooden block.
• Switch on the 12 V supply. (The lamp will not light, since the circuit is incomplete.)

f Position the two bare wires as shown in the illustration. They should not quite touch at this stage. Adjust the separation of the two bare wires by moving the support block nearer to the electromagnet or further from it.

g When the switch is closed, the first circuit is complete and the blade will be attracted downwards. The two bare wires will touch. This will complete the second circuit, so that the lamp will light.

h Open the switch. The first circuit is broken, and so the electromagnet is no longer energized. The blade moves upwards and the second circuit is broken. The lamp goes out.

Teaching notes

1 A relay is an automatic electric switch. This model is intended to make it easier for students to understand how a relay operates: switching one circuit causes one or more other circuits to be switched. A small current sent through the relay's coil makes the relay switch on (or switch off) a big current. Or a small current may make a different relay connect up several other circuits.

2 A relay hands a switching signal on from one circuit to another. That is why it is called a relay, after a relay race in which one runner hands the torch on to the next. Relays were once found by the thousand in telephone exchanges. There are huge relays in a power station, and controlling relays in many factories with automated manufacturing systems. But these functions are increasingly performed by solid-state electronic devices.

This experiment was safety-checked in July 2007

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Page last updated on 09 November 2011