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the principle on which a dynamo produces electricity.
Electricity flowing through a conductor may be likened to
water flowing through a pipe. The amount of electricity that
flows is measured in units called amperes. The voltage is
measured in volts. Just as a thin pipe will not allow as much
water to flow through it as a large one, so thin conductor will not
allow as much electricity to flow as a thick one. The restriction to
the flow caused by the conductor is known as its resistance. A
unit of resistance is called an ohm. The resistance of a conductor
will also vary according to the material of which it is made.
Copper has a much lower resistance than other common metals.
It is a very good conductor.
Current, voltage and resistance bear a definite relationship
to one another. Ohm’s law tells us that current equals voltage
divided by resistance. The current and the voltage multiplied
together determine the power, the rate at which the electrical
energy is used up by being converted to other forms of energy. A
unit of power is watt.
To simplify the figures used for great power, abbreviations
are used. One thousand watts is a kilowatt and one million watts
is a megawatt. Very sensitive instruments can measure a
milliwatt, which is a thousandth part of a watt. A millionth part of
a watt is a microwatt. Electrical appliances are described
according to the amount of power they consume; we talk of a 60
watt lamp, a 2 kilowatt fire.
In some pieces of electrical apparatus it is sometimes
necessary to store an electrical charge, to build it up and hold it
for some time. This is done by means of a capacitor, sometimes
called a condenser.’ A simple capacitor is made from two metal
plates, called electrodes,’ which are separated by an insulating
material such as air, paper, or mica, called the dielectric.

III

The unit of capacitance is called a farad. This is a relatively
large unit. Many capacitors used in electronic apparatus are rated
in microfarads, millionth part of a farad. It is sometimes

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