the top of the plates. In process of time the liquid will be found to evaporate; this should be made up by a little clean rain-water, or preferably distilled water.
Switch.—It will be remembered that each battery has two free ends or 'terminals.' From one of these a wire is led to an 'interrupter' or 'switch.' This switch may take one of many forms. The effect of the apparatus is to easily and quickly complete or interrupt the circuit at any desired time in the same way that a tap is used to turn off water.
Induction Coil. From the switch another wire is taken to an apparatus known as the 'induction coil.' The function of this is to greatly intensify the current. On a well-known electrical principle, the current, which is of low pressure (four volts), when it enters the coil, is intensified to a very great degree. The current being required to jump across a considerable gap inside the combustion chamber, a much greater pressure than four volts is essential.
To explain the method of connecting the coil with the battery and engine, it will be necessary to give a brief description of the coil. In the centre lies a bundle of iron wires, known as the 'core,' around which is wound a quantity of thick copper wire, insulated with silk or cotton. This wire is in one piece, and known as the 'primary' winding. On the top of this layer lie laps of very fine wire, likewise carefully insulated. This is known as the 'secondary' circuit. There is also usually contained in the same case an arrangement called a 'condenser,' which we need not describe. Although the two circuits are quite distinct from one another, a current of electricity passing round the primary and suddenly interrupted by means hereafter described will 'induce' a current in the 'secondary' of very great pressure. The ends of the two windings are led to the outside of the case, and terminate in screws or binding posts. These terminals are usually stamped with letters to indicate the method of connection. As many French coils are in use, it may help the novice to mention that the letter p stands for battery, m for commutator, b sparking plug, while in
