• Every time the contacts break, a small emf is induced in the primary coil. This would eventually burn out the contacts if the capacitor were not absorbing some of the energy.
adjustable terminal T1 spring terminal T2
soft iron pole
spark gap S
capacitor C battery B
T1 T2
C S C2 C1
secondary coil C2
soft iron core I STS
As induction coils are capable of producing large pulses of voltage, they are used in spark plugs, electric fences and anywhere a smaller voltage supply is required to produce a large-voltage pulse.
In an induction coil:
• the soft iron core enhances the emf induced • the terminal contacts and spark gap can be adjusted
• the primary coil is a thicker wire of few turns • the secondary coil is much thinner wire of many thousands of turns. This allows for a much higher emf to be induced proportionally
• the emf induced when the contacts close is not important as it is much smaller owing to a slowly rising magnetic field.
Alternating current
As we learned in Chapter 23, current can be d.c. or a.c. Alternating current reverses direction within a set period. The mains supply in Ireland is 50 Hz. This means direction changes every 1/100th second. It requires a full period to have 1 cycle. Therefore there are 50 cycles a second, or 50 Hz, in the supply.
The 50 Hz waveform is ‘sinusoidal’ (sine) in nature. This sine wave shows positive and negative motion of current.
In order to produce alternating current, we need an a.c. generator. This is similar to a d.c. generator and operates under the same principles. By replacing the commutator with two slip rings, we can modify a d.c. generator to become an a.c. generator. The generator still contains a coil within a magnetic field but the slip rings now rotate with the armature of the coils and the carbon brushes press against them, delivering a contact for current (Fig. 33.16).
388 FUSION
FIG. 33.15 A spark can be seen to be produced in this induction coil
I
primary coil C1
FIG. 33.14 An induction coil and associated circuit diagram B
