If, now, these two wires B C are put into circuit with each other, as illustrated by the wires D, we shall have a form of battery which will have the voltage equal to the voltage of one cell multiplied by the cells in either battery 1 or 2. This is 1-1/2 volts × 3, equal to 4-1/2. The amperage, on the other hand, is found by that of one cell multiplied by the number of batteries. This is 25 amperes × 2, equal to 50. This, if well understood, will enable the user, for instance, to strengthen a battery, where it is weak, by connecting it up in series multiple, instead of in parallel. Naturally, the cells in the series should be of equal strength and should be frequently tested, to find where the weakness is. If the combined amperage is below the minimum, considering the time it has been in use, it is possible the cause is due to a weak cell, which takes from others, instead of giving. This should be replaced. STORAGE BATTERIES.--The matter pertaining to these batteries is fully set forth in connection with Electric vehicles, in a subsequent chapter. Primary as well as storage batteries may be used for ignition purposes, the object being to obtain a form of battery which shall have a constant and reliable output, and give a reasonable service in point of time. THE SPARKING METHODS.--Automobiles are equipped with either the _low_ or the _high_ tension system. Any circuit having a small voltage is termed _low tension_, to distinguish it from a _high tension_, or high voltage. When a current passes along a conductor, no visible effect is produced, unless the voltage should be too great for the carrying wire. In that case it will heat the conductor to redness and thus enable the eye to see it. The heat is thus caused by _resistance_. AIR RESISTANCE.--Air has resistance, the same as all other substances. It is, in fact an absolute non-conductor, so that with an ordinary current, such as is used for electric lighting, the separated ends of a conductor may be placed very close together and the current would not leap across. MAKE AND BREAK SPARK.--On the other hand, even with the weakest current, if the two ends are brought into contact, and then separated, a spark will follow, due to the flow of the current which is interrupted at the breaking of the contact, and the effort of the current to keep on flowing through the wire. This is called the low tension system, or the _make_ and _break_ method of ignition, where the act of breaking the circuit produces the spark and ignites the charge. _The high tension system_, on the other hand, depends on producing a current of sufficient pressure to be able to make the current leap across the small gap which is formed between the ends of the conductor.