Invented in 1851, the induction coil, shown in
Figure 1, is a piece of apparatus used to produce very high voltages from a low voltage d.c.
supply.
When the switch (S) is closed a current flows through the primary coil this
magnetises the soft iron rods in the core of the coil. These then attract the soft iron
armature(A) which therefore breaks the contact at C. The primary current falls to zero very
quickly. This sudden drop in current induces a large voltage in the secondary coil that has a
large number of turns. As soon as the contact is broken the magnetic field of the rods
disappears, the armature springs back and makes the contact at C once again. The process
then repeats itself.
Voltages of many tens of thousands of volts can be made from a 6-
12 V input.
Figure 2 shows the use of the induction coil in the ignition circuit of a car
— probably its most important use today.
