The quantity of water flowing along the pipe is measured, as electricity, in amperes. As the quantity of water passing in a given time is regulated by the size of the pipe and its own pressure, so the quantity of electricity is also regulated. A conductor of electricity offers resistance to the flow of current according to its sectional area and the material of which it is composed, this resistance being expressed in ohms. The greater the voltage and lower the resistance, the more current. This law, and its kindred applications, are expressed as follows:
C = E/R.
C is current in amperes, E electromotive force in volts, and R resistance in ohms.
Thus a wire with a resistance of 50 ohms would pass 2 amperes with an electromotive force of 100 volts. To find resistance when other two factors are known, the formula is
R = E/C.
In selecting a battery for work, regard must be made to the current required, and its period of flow. For energizing a gas lighting primary coil, the current must be large, but is only required occasionally, the battery standing idle for long periods. In this case the class called open circuit cells are preferable, as they contain no strong acids and do not deteriorate to any extent when not in use. Of such class is the Leclanche-Samson, Monarch, carbon cylinder, and most so-called dry cells. As the resistance in a conductor affects the current flow, so it does in a battery cell; the internal resistance of a battery is determined by its size, proximity of the elements, etc. Cells with small zincs and porous cups are of high internal resistance, those with large sheet zincs and big carbon surfaces, of low internal resistance. As the primary coil used in gas lighting is never much over one ohm, a cell of low internal resistance should be selected. But as the wires leading to the burners must be taken into account, a number of cells should be used to produce enough electromotive force to overcome the added resistance. Now battery cells can be arranged in a variety of ways—in series for higher electromotive force, and in multiple—for greater current.
Fig. 50.
Fig. 50 represents the series arrangement; here the zinc of one cell is connected to the carbon of the next; this adds the electromotive forces together and thus gives greater ability to overcome resistance, but it also adds together the resistance of each cell. Thus 4 cells, each 2 volts and of one-half ohm internal resistance, would, in series, have an E. M. F. of 8 volts and an internal resistance of 2 ohms, current 4 amperes. Fig. 51 shows four cells in multiple, the zinc of each cell and the carbons of each cell are connected. Here the result would be but 2 volts, but the internal resistance would be only one-quarter, viz: one-eighth of an ohm, current 16 amperes.
