As we have explained to you, closed-circuit batteries are used for
producing incandescent electric lights in small numbers, as well as
for running motors.
To operate incandescent lights, a number of batteries connected
together are used. The number used depends upon the pressure which the
lamps require to make them give the required light. We will now explain
how the batteries are connected together for this purpose.
[Illustration: Fig. 32]
Suppose you wished to light an incandescent lamp of, say, three
candle-power, which required six volts. We would take three
closed-circuit batteries which would each give two volts, and connect
by a piece of wire the zinc of the first to the carbon of the second,
and the zinc of the second to the carbon of the third, as shown in the
sketch. (Fig. 32.)
We would then attach a wire to the carbon of the first and one to the
zinc of the third, and there would be six volts in these two wires,
which would light up one six-volt lamp nicely.
This is called connecting in series, or for intensity.
Now if each of these cells gave ten ampères alone, the three will only
give ten ampères together when they are connected in series.
If our lamp only required one ampère, you would naturally think that
ten similar lamps put on the wires would give as good light as the one,
but that is not so.
Although you might light up two lamps, the pressure would drop and the
lights would become less brilliant if you put on the whole number. So,
if we wished to put on the whole ten lights we would connect another
battery and thus increase the pressure, which would probably make these
ten lamps burn brightly.
These rules hold good for connecting any number of batteries for lamps
of any number of volts--that is to say, there should be calculated
about two volts for each cell and an allowance made for drop in
pressure.