PURPOSE: Counterintuitive demonstration of how the primary current in a transformer increases when the secondary load increases.

DESCRIPTION: A 15-watt light bulb in series with the primary of a transformer is used as the "sensor" of primary current. Switching on a small bulb across the secondary makes the primary sensor bulb glow more brightly. Before switching on the secondary bulb, ask your students what will happen to the bulb in the primary circuit. The picture at the right shows the secondary circuit switched on (switch at lower left) so the small secondary bulb is glowing and the primary bulb gowing more brightly.

Ask your students what will happen to the primary sensor bulb when the secondary bulb is removed. After re-connecting, ask your students what will happen to the secondary bulb when the primary bulb is removed.

SUGGESTIONS: See Question of the Week #55 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 5K30.60)

EQUIPMENT: Demountable transformer with two 500-turn primary coil and 46-turn secondary coil, 15-Watt frosted light bulb, 4-volt light bulb on switched secondary socket.

SETUP TIME: 5 min.