Experiment 25: Magnetism
236 Chapter 5
Experiment 25: Magnetism
This experiment should be a part of any school science class, but even if you
remember doing it, I suggest that you do it again, because setting it up takes
only a matter of moments, and it’s going to be our entry point to a whole new
topic: the relationship between electricity and magnetism. Quickly this will
lead us into audio reproduction and radio, and I’ll describe the fundamentals
of self-inductance, which is the third and final basic property of passive com-
ponents (resistance and capacitance being the other two). I left self-inductance
until last because it’s not very relevant to the experiments that you’ve done
so far. But as soon as we start dealing with analog signals that fluctuate, it
becomes essential.
You will need:- Large screwdriver.
- 22-gauge wire (or thinner). Quantity: 6 feet.
- AA battery.
Procedure
This couldn’t be simpler. Wind the wire around the shaft of the screwdriver,
near its tip. The turns should be neat and tight and closely spaced, and you’ll
need to make 100 of them, within a distance of no more than 2 inches. To fit
them into this space, you’ll have to make turns on top of previous turns. If the
final turn tends to unwind itself (which will happen if you’re using stranded
wire), secure it with a piece of tape. See Figure 5-11.
Now apply a battery, as shown in Figure 5-12. At first sight, this looks like a
very bad idea, because you’re going to short out your battery just as you did in
Experiment 2. But by passing the current through a wire that’s coiled instead
of straight, we’ll get some work out of it before the battery expires.
Put a small paper clip near the screwdriver blade, on a soft, smooth surface
that will not present much friction. A tissue works well. Because many screw-
drivers are already magnetic, you may find that the paper clip is naturally
attracted to the tip of the blade. If this happens, move the clip just outside
the range of attraction. Now apply the 1.5 volts to the circuit, and the clip
should jump to the tip of the screwdriver. Congratulations: you just made an
electromagnet.Figure 5-12. A schematic can’t get much simpler than this.FundAmentAls
A two-way relationship
Every electric motor that was ever
made uses some aspect of the
relationship between electricity
and magnetism. It’s absolutely
fundamental in the world around
us. Remember that electricity can
create magnetism:
When electricity flows through a
wire, it creates a magnetic force
around the wire.
The principle works in reverse: mag-
netism can create electricity.
When a wire moves through a
magnetic field, it creates a flow of
electricity in the wire.
This second principle is used in
power generation. A diesel engine,
or a water-powered turbine, or a
windmill, or some other source of
energy either turns coils of wire
through a powerful magnetic
field, or turns magnets amid some
massive coils of wire. Electricity is
induced in the coils. In the next
experiment, you’ll see a dramatic
mini-demo of this effect.Figure 5-11. Anyone who somehow missed
this most basic childhood demo of electro-
magnetism should try it just for the fun of
proving that a single AA battery can move
a paper clip.