High and Low Voltage.—A high tension current,—that is, one having a high voltage, will leap across a gap, whereas a low voltage must have an easy path. When the ends of a wire in a circuit are separated, air acts as a perfect insulator between them, and the slightest separation will prevent a low current from jumping across.
This is not the case with a high tension current, where it will leap across and produce the flash known as the jump spark.
Low Tension System.—Two distinct types of ignition have grown out of the voltage referred to, in which the make and break system uses the low tension, because of its simplicity in the electrical equipment.
Disadvantages of the Make and Break.—There is one serious drawback to the extended use of this system, and that is the necessity of using a moving part within the cylinder, to make and break the contact in the conductor, as it is obvious that this part of the mechanism must be placed within the compressed mixture in order to ignite it.
Amperes.—A current is also measured by amperes,—that is, the quantity flowing. A large conductor will take a greater quantity of current than a small one, just as in the case of water a large pipe will convey a greater amount of the liquid.
Resistance.—All conductors offer resistance to the flow of a current, and this is measured in Ohms. The best conductor is silver and the next best is copper, this latter material being used universally, owing to its comparative cheapness.
Iron is a relatively poor conductor. Resistance can be overcome to a certain extent, however, if a large conductor is used, but it is more economical to use a small conductor which has small resistance, like copper, than a heavy conductor, as iron, even though pound for pound the latter may be cheaper.
Direct Current.—There are two kinds of current, one which flows in one direction only, called the Direct. It is produced in a dynamo which has a pair of commutator brushes so arranged that as the armature turns and its wires move through the magnetic fields of a magnet, and have direction of the current alternate, these brushes will change the alternations so the current will travel over the working conductors in one direction only.
Primary and secondary batteries produce a direct current. These will be described in their appropriate places.