THE COMMON PRIMARY CELL.--The most common form of primary cell contains sulphuric acid, or a sulphuric acid solution, as the electrolyte, with zinc for the _anode_, and carbon, instead of copper, for the _cathode_. The ends of the zinc and copper plates are called _terminals_, and while the zinc is the anode or positive element, its _terminal_ is designated as the positive pole. In like manner, the carbon is the negative element or cathode, and its terminal is designated as negative pole. Fig. 21 will show the relative arrangement of the parts. It is customary to term that end or element from which the current flows as positive. A cell is regarded as a whole, and as the current passes out of the cell from the copper element, the copper terminal becomes positive. [Illustration: _Fig. 21._ PRIMARY BATTERY] BATTERY RESISTANCE, ELECTROLYTE AND CURRENT.--The following should be carefully memorized: A cell has reference to a single vessel. When two or more cells are coupled together they form a _battery_. _Resistance_ is opposition to the movement of the current. If it is offered by the electrolyte, it is designated "Internal Resistance." If, on the other hand, the opposition takes place, for instance, through the wire, it is then called "External Resistance." The electrolyte must be either acid, or alkaline, or saline, and the electrodes must be of dissimilar metals, so the electrolyte will attack one of them. The current is measured in amperes, and the force with which it is caused to flow is measured in volts. In practice the word "current" is used to designate ampere flow; and electromotive force, or E. M. F., is used instead of voltage. ELECTRO-MAGNETIC ELECTRICITY.--The third method of generating electricity is by electro-magnets. The value and use of induction will now be seen, and you will be enabled to utilize the lesson concerning magnetic action referred to in the previous chapter. MAGNETIC RADIATION.--You will remember that every piece of metal which is within the path of an electric current has a space all about its surface from end to end which is electrified. This electrified field extends out a certain distance from the metal, and is supposed to maintain a movement around it. If, now, another piece of metal is brought within range of this electric or magnetic zone and moved across it, so as to cut through this field, a current will be generated thereby, or rather added to the current already exerted, so that if we start with a feeble current, it can be increased by rapidly "cutting the lines of force," as it is called.