A number of cells coupled together form a battery, and different methods of coupling are used to get different results. In addition to the resistance of the circuit outside the cell, the cell itself offers an internal resistance, and part of the electro-motive force is used up in overcoming this resistance. If we can decrease this internal resistance we shall have a larger current at our disposal, and one way of doing this is to increase the size of the plates. This of course means making the cell larger, and very large cells take up a lot of room and are troublesome to move about. We can get the same effect however by coupling. If we connect together all the positive terminals and all the negative terminals of several cells, that is, copper to copper and zinc to zinc in Daniell cells, we get the same result as if we had one very large cell. The current is much larger, but the electro-motive force remains the same as if only one cell were used, or in other words we have more amperes but no more volts. This is called connecting in “parallel,” and the method is shown in Fig. 8. On the other hand, if, as is usually the case, we want a larger electro-motive force, we connect the positive terminal of one cell to the negative terminal of the next, or copper to zinc all through. In this way we add together the electro-motive forces of all the cells, but the amount of current remains that of a single cell; that is, we get more volts but no more amperes. This is called connecting in “series,” and the arrangement is shown in Fig. 9. We can also increase both volts and amperes by combining the two methods. [Illustration: FIG. 9.--Cells connected in Series.] A voltaic cell gives us a considerable quantity of electricity at low pressure, the electro-motive force of a Leclanché cell being about 1½ volts, and that of a Daniell cell about 1 volt. We may perhaps get some idea of the electrical conditions existing during a thunderstorm from the fact that to produce a spark one mile long through air at ordinary pressure we should require a battery of more than a thousand million Daniell cells. Cells such as we have described in this chapter are called primary cells, as distinguished from accumulators, which are called secondary cells. Some of the practical applications of primary cells will be described in later chapters.