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.