A model electric bell
A clever application of feedback: a switch that opens and closes due to temporary magnetism in a current-carrying coil of wire.
Apparatus and materials
For each student group
C-core, laminated iron
Copper wire, PVC-covered, 150 cm with bare ends
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.
a Take one iron C-core. Wind twenty turns of PVC-covered copper wire round one arm and connect one end to one of the 1 V d.c. 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 to other terminals.
c Attach a mass (e.g. some Plasticine) to the free end of the blade, to slow its vibrations to about four vibrations per second.
d Put the C-core under the projecting blade, but not quite touching it.
e Connect one end of a short length of wire to the other DC terminal of the supply. Tape the other end so that the bare wire protrudes along the length of the blade.
f Position the unconnected end of the coiled wire so that it makes gentle contact with the shorter wire.
g When the supply is switched on, the blade will be attracted downwards. This will break the circuit, so that the blade springs upwards again, re-completing the circuit.
This model makes it easier for students to understand the automatic switching which is at the heart of operation of the traditional electric bell.
This experiment was safety-checked in June 2007