Electric bell model

Product Details : 

Electric bell model for dc

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
• Hacksaw blade
• Plasticine
• Copper wire, PVC-covered, 150 cm with bare ends
• Adhesive tape
• Power supply, low-voltage
• 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.
  
• Read our standard health & safety guidance
  
• The blades must be demagnetised before each lesson because they could display an assortment of magnetic poles along their lengths.
  

Procedure

• 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.
• 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.
• Attach a mass (e.g. some Plasticine) to the free end of the blade, to slow its vibrations to about four vibrations per second.
• Put the C-core under the projecting blade, but not quite touching it.
• 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.
• Position the unconnected end of the coiled wire so that it makes gentle contact with the shorter wire.
• 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.