Experiment 28: Making a Coil React
246 Chapter 5
Experiment 28: Making a Coil React
A capacitor will absorb some DC current until it is fully charged, at which point
it blocks the flow. There’s another phenomenon that I haven’t mentioned so
far, which is the exact opposite of capacitance. It’s known as self-inductance,
and you find it in any coil of wire. Initially it blocks DC current (it reacts against
it), but then its opposition gradually disappears. Here are a few definitions:
Resistance
Constrains current flow and drops voltage.
Capacitance
Allows current to flow initially and then blocks it. This behavior is properly
known as capacitive reactance.
Self-Inductance
Blocks the flow of current initially and then allows it. This is also often re-
ferred to as inductive reactance. In fact, you may find the term “reactance”
used as if it means the same thing, but since self-inductance is the correct
term, I’ll be using it here.
In this experiment, you’ll see self-inductance in action.
You will need:- LEDs, low-current type. Quantity: 2.
- Spool of hookup wire, 26-gauge, 100 feet. Quantity: 1.
- Resistor, 220Ω, rated 1/4 watt or higher. Quantity: 1.
- Capacitor, electrolytic, 2,000 μF or larger. Quantity: 1.
- SPST tactile switch. Quantity: 1.
Procedure
Take a look at the schematic in Figure 5-33. At first it may not make much
sense. The curly symbol is a coil of wire—nothing more than that. So appar-
ently the voltage will pass through the 220Ω resistor, and then through the
coil, ignoring the two LEDs, because the coil obviously has a much lower resis-
tance than either of them (and one of them is upside-down anyway).
Is that what will happen? Let’s find out. The coil can be a spool of 100 feet of
26-gauge (or smaller) hookup wire, although the spool of magnet wire listed
in Experiment 25 will work better, if you have that. Once again, you will need
access to both ends of the wire, and if the inner end is inaccessible, you’ll need
to rewind the coil, leaving the end sticking out.
Now that you have a coil, you can hook it up on your breadboard as shown in
Figure 5-34, where the green circle is a tactile switch and the two circular red
objects are LEDs. Make sure that you use low-current LEDs (otherwise, you
may not see anything) and make sure that one of them is negative-side-up,
positive-side-down and the other is positive-side-up, negative-side-down.
Also, the 220Ω resistor should be rated at 1/4 watt or higher, if possible (see
the following caution).12V
DC220
D1
D2
Figure 5-33. In this demonstration of self-
inductance, D1 and D2 are light-emitting
diodes. When the switch is closed, D1
flashes briefly because the coil obstructs
the initial flow of electricity. When the
switch is opened, D2 flashes as the col-
lapsing magnetic field induced by the coil
releases another short burst of current.