New trends in physics teaching, v.4; The ... - unesdoc - Unesco

String and tape experiments
For a large audience, the strip can be folded and laid loosely across an overhead projector,
taping the ends of the strip to the edge of the projection plate. The motion is not as large, because
of friction with the transparent plate, but the magnification makes it easily visible. To show that
currents in the same direction attract, the top ends of the folded strip are taped together and
attached to one terminal of the supply, and the fold at the bottom end attached by a separate
foil strip to the other terminal of the supply. This time, the two parts of the strip wil move
together as shown. It is possible to use either a d.c. or a.c. supply in this experiment, but connecting
directly across the mains supply is definitely not recommended - you wil either blow a fuse
or melt the aluminium!
A current balance
Many more experiments are possible if a coil of wire is available. The kind called ‘Magnet wire’
(number 22 gauge insulated is good) is probably the best to buy. Here is a simple experiment
designed by E. J. Wenham for the British Nuffield Physics course [2]. It requires a small alnico
magnet (about 1.2 cm long and 0.4 cm square) a drinking straw, a needle and a small channel
made of card such as the outside of a match box, to support the needle.
Fasten the magnet near to the end of the drinking straw with about 4 cm of sticky tape (cut
to half width) (see figure 30). Balance the straw across your finger to find its centre of gravity,
and stick the needle through the straw about 1 mm farther away from the magnet. Wind a coil
of wire about 25 mm diameter with about 20 turns and fix it with sticky tape to the table top
close to the end of the channel, which is made by cutting the match box top so that it stands
about 5 mm higher than the top of the coil (figure 3 1).
magnet 03
Figure 30. Making the magnet assembly for a current balance [2].
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