Make Electronics

What Next? 237

Experiment 25: Magnetism

theory

Inductance

When electricity flows through a wire, it creates a magnetic
field around the wire. Because the electricity “induces” this
effect, it is known as inductance. The effect is illustrated in
Figure 5-13.

Figure 5-13. When the flow of electricity is from left to right
along this conductor, it induces a magnetic force shown by the
green arrows.

The field around a straight wire is very weak, but if we bend
the wire into a circle, the magnetic force starts to accumu-
late, pointing through the center of the circle, as shown in
Figure 5-14. If we add more circles, to form a coil, the force
accumulates even more. And if we put a magnetic object
(such as a screwdriver) in the center of the coil, the effective-
ness increases further.

Figure 5-14. When the conductor is bent to form a circle, the cu-
mulative magnetic force acts through the center of the circle,
as shown by the large arrow.

Here’s an approximated formula showing the relationship

between the diameter of the coil, the width of the coil from

end to end, the number of turns, and its inductance. The

letter L is the symbol for inductance, even though the unit

is the Henry, named after an American electrical pioneer

named Joseph Henry:

L (in microHenrys) =

[(D × D) × (N x N)] / [(18 × D) + (40 × W)]

(Approximately)

In this formula, D is the diameter of the coil, N is the number

of turns, and W is the width of the coil from end to end. See

Figure 5-15. Here are three simple conclusions from this

formula:

• Inductance increases with the diameter of the coil.


• Inductance increases with the square of the number of
  turns. (In other words, three times as many turns create
  nine times the inductance.)


• If the number of turns remains the same, inductance is
  lower if you wind the coil so that it’s slender and long,
  but is higher if you wind it so that it’s fat and short.

W = Width

of coil

D=Diameter

of coil

N=Number

of turns of

wire

Figure 5-15. The inductance of a coil increases with its diameter

and with the square of its number of turns. If all other param-

eters remain the same, reducing the width (the distance from

end to end) by packing the turns more tightly will increase the

inductance.