=275. Internal Resistance of a Voltaic Cell.=--The current produced by a voltaic cell is affected by the resistance that the current meets in passing from one plate to another through the liquid of the cell. This is called the _internal resistance_ of the cell. A Daniell cell has several (1-5) ohms internal resistance. The resistance of dry cells varies from less than 0.1 of an ohm when new to several ohms when old. If cells are joined together their combined internal resistance depends upon the method of grouping the cells. [Illustration: FIG. 256.--The four cans exert four times the water pressure that one can will exert.] =276. Cells Grouped in Series and in Parallel.=--When in _series_ the copper or carbon plate of one cell is joined to the zinc of another and so on. (See Fig. 251.) The effect of connecting, say four cells, in series may be illustrated by taking four cans of water, placed one above another. (See Fig. 256.) The combined water pressure of the series is the sum of the several pressures of the cans of water, while the opposition offered to the movement of a quantity of water through the group of cans is the sum of the several resistances of the cans. In applying this illustration to the voltaic cell, we make use of Ohm's law. Let _E_ represent the e.m.f. of a single cell, _r_ the internal resistance of the cell, and _R_ the external resistance or the resistance of the rest of the circuit. Consider a group of cells in series. If _n_ represents the _number_ of cells in _series_, then Ohm's law becomes _I_ = _nE_/(_nr_ + _R_). Cells are grouped in _series_ when large E.M.F. is required to force a current through a large external resistance such as through a long telegraph line. Cells are connected in _parallel_ when it is desired to send a large current through a small external resistance. To connect cells in parallel all the copper plates are joined and also all the zinc plates. (See Fig. 257.) To illustrate the effect of this mode of grouping cells, suppose several cans of water are placed side by side (Fig. 258). It is easily seen that the pressure of the group is the same as that of a single cell, while the resistance to the flow is less than that of a single cell. Applying this reasoning to the electric circuit we have by Ohm's law the formula for the current flow of a group of _n_ cells arranged in parallel _I_ = _E_/((_r/n_) + _R_). [Illustration: FIG. 257.--Four cells connected in parallel.] [Illustration: FIG. 258.--The water pressure of the group in parallel is the same as that of one.]