_The Circuit._--In connection with the manipulation of batteries, there is one important item to consider, viz. the _resistance_ in the _circuit_, which may be divided into _external_ and _internal_. _Resistances._--The _external_ resistance in practice is that which exists in the conducting line, and the various instruments connected with it. The _internal_ resistance is that which exists in the battery itself. All known conductors oppose a sensible _resistance_ to the passage of an electric current, and the strength of the current, or in other words, the quantity of electricity passing per second from one point to another, when a constant difference of potentials is maintained between them, depends on the _resistance_ of the wire on the conductor joining them. A bad conductor does not let the electricity pass so rapidly as a good conductor, that is, it offers more _resistance_. Resistance in a wire of constant section and material is _directly_ proportional to the _length_, and _inversely_ proportional to the _area of the cross section_. The electrical resistance of a conductor must not be considered as analogous to mechanical resistance, such as the friction which water experiences in passing through a pipe, for this frictional resistance _is not_ constant when different quantities of water are being forced through the pipe, whereas electrical resistance is constant whatever quantity of electricity be forced through the conductor. _Application of Ohm's Law._--_Ohm's law_, which governs the strength of the current, is expressed by the equation C = E / R or R = E / C or E = CR. Where C is the strength of the current; E is the E.M.F. or difference of potentials; and R is the resistance of the circuit. */ In words, _Ohm's law_ means that the strength of the current is _directly_ proportional to the E.M.F., and _inversely_ proportional to the resistance of the circuit. As before stated, the resistance of the circuit consists of an _external_ and an _internal_ resistance, therefore when these resistances are separately considered, the equation C = E / R must be converted into C = E / (_x_ + _r_), where _x_ is the external, and _r_ the internal, resistance. The resistance of the battery or the _internal resistance_ depends on the size of the plates and the distance between them, that is, it is _directly_ proportional to the distance, and _inversely_ proportional to the size. The _electro-motive_ force of a battery is dependent generally on the number of cells joined in _series_, and not on the _size_ of the plates. The cells of a battery may be joined in two ways, as follows:-- 1. In series: that is, by connecting the negative element of one cell to the positive element of another, and so on. 2. In multiple arc: that is, by connecting negative to negative, and positive to positive; which is the same as increasing the size of the cells.