# A model buzzer

##### Class practical

Electromagnets of all shapes and sizes can be found doing useful jobs in machinery such as buzzers, bells and relays.

#### Apparatus and materials

For each student group

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

C-core, laminated iron

#### 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 Take one iron C-core. Wind twenty turns of PVC-covered copper wire round one arm and connect to the 1 V AC terminals of the low-voltage power supply.

b Clamp a strip of steel such as a hacksaw blade under a spare terminal of the low-voltage supply, taking care that it does not accidentally short-circuit other terminals.

c Put the C-core under the projecting blade, but not quite touching it. When the supply is switched on, you will feel distinct vibrations in the blade.

d Adjust the length of the blade to give the largest amplitude of vibration.

#### Teaching notes

1 The length of the blade can be tuned to resonance and will give a noticeable amplitude. The length is critical and will depend on the blade.

Photograph courtesy of Mike Vetterlein

2 The vibrating blade can be allowed to hit against a bell or tin can to provide a ringing tone, if the teacher can stand all the noise - which is twice the frequency of the supply.

3 Rather than supporting the blade under a spare terminal of the power supply, you could use a wooden block to support it, as shown below.

4 The principle of polarizing may be shown by using Magnadur (ceramic) magnets as shown. In this case, the blade is attracted continuously to the electromagnet. This gets stronger and weaker with the frequency of the mains supply. How does the pitch of the note change?

5 A number of similar models could be constructed to illustrate a range of practical applications of electromagnets: for example, the buzzer, the bell, telephone earpieces, control solenoids. These might be given to the students as optional project work for them to see what they can achieve using equipment of this sort.

This experiment was safety-checked in July 2007