Current producers that convert chemical or mechanical energy into electrical energy are called primary generators, and are represented by the primary battery and dynamo. The above methods are used for generating current for either the high or low tension systems. Electricity may also be produced by friction, but as such current is without heat value it is not used for ignition purposes. Electricity produced by friction is called static electricity. Primary and storage batteries always deliver a direct or continuous current of electricity, that is a current which flows continually in one direction. Dynamos are usually made to furnish a direct current, but can be built to deliver either direct or alternating. Alternating current, unlike the continuous current, changes the direction of its flow periodically; flowing first in one direction and then in the other, the flow alternating in equal periods of time. Magnetos being a special form of dynamo can furnish either class of current, but with few exceptions are built for generating alternating current. Either current may be used for ignition purposes for either high or low tension systems. Alternating current has several advantages not possessed by the continuous current, when used for ignition purposes. The principal advantages are: 1. Alternating current does not transfer the electrode metal of contact points, and consequently causes less trouble with vibrators and “make” and “break” ignitors. 2. Magnetos generating alternating current are less complicated, have fewer parts to get out of order, and are cheaper to keep in repair. 3. Alternating current is not liable to burn out spark coils or overheat with an excessive voltage. 4. Alternating current generators can be used at any speed without the use of governors. [Illustration: 43-a. The Esselbé Rotary Aero Motor. Four Pistons are Contained in the Ring Shaped Cylinder at the Left Which are so Connected with Cranks and Gears in the Gear Box that the Pistons and the Cylinder Rotate in Opposite Directions. As the Pistons Rotate they also Oscillate Back and Forth in Regard to One Another, so that the Working and Compression Strokes are Performed. From Aero London. ] When installing an ignition system give due consideration to the reliability of the source of current. The gas engine is no more reliable than its source of current. Failure of the current means the failure of the engine. (80) Primary Batteries.