cells are joined in series the amperage of the set is equal to that of but one cell. If it is desired to obtain greater amperage, the cells are joined in multiple, which means that all terminals of similar polarity are joined together. A battery of four dry cells connected in this manner would deliver a current of about eighty amperes.

When dry batteries are used for ignition purposes, they are generally coupled together in a series-multiple connection to obtain the proper voltage and current strength. The dry battery has a number of advantages, chief among which are its cheapness, ease of installation, compactness and simplicity. It has the disadvantage of being limited in capacity and not suited for continuous work, which it shares with all other forms of primary battery. When dry cells are exhausted there is no method of renewing them to efficiency and they must be replaced. This is not a serious factor, however, as an entire new set of six cells will cost but $1.50.

The storage battery is a chemical current producer that is capable of being recharged when it is exhausted by passing a current of electricity through it in a reverse direction to that of the current given out. Storage batteries are composed of elements of practically the same material and can only become active when a current of electricity is passed through them. The materials generally used are grids of lead filled with a paste composed of lead oxide. When the current of electricity passes through these plates, they become enough different in nature so that a difference of electrical condition exists between them, and when the cell is fully charged, a current may be drawn from it in just the same way as from a primary battery.

Storage batteries have the advantage that they may

be used for continuous current production, and as they may be recharged when exhausted, it is not necessary to replace them with new members when they will no longer produce current. The storage battery is called a "secondary cell" because it can only give out energy after a current of electricity has passed through it, whereas a primary battery in good condition will produce electricity as soon as it is completed. The storage battery uses an electrolyte composed of dilute sulphuric acid and water, while a dry battery uses an alkaline electrolyte composed largely of sal-ammoniac and chloride of zinc.

The average form of storage battery used for ignition purposes is really composed of three cells, which are placed in a common carrying case of wood or hard rubber. The connection between the cells is made by plates of lead which are burned to the elements, leaving but two terminals free, one of which is a negative member while the other leads from the positive plates. To prevent spilling of the electrolyte the top of the cell or battery is sealed with a hard rubber plate over which is poured a pitch and rosin compound. The electrolyte is renewed through a small vent in each cell which is covered by a removable hard rubber cap. These vents also allow for the escape of the gases evolved when the cell is being charged or when it is delivering a current of electricity. The average ignition battery has a capacity of 60 to 80 ampere hours and a current pressure of about 6.6 volts when fully charged.

Functions of Induction Coil.-The current obtained from a dry or storage battery is not sufficiently powerful to leap the gap that exists between the points of a spark plug utilized in the high tension ignition system. This space offers a resistance to the passage

of the current that requires a pressure of several thousand volts to overcome, and as the ordinary ignition battery produces but six volts it will be apparent that unless the current strength is increased it could not overcome the resistance of the air gap between the spark plug points. A simple device known as an "induction coil" is employed to transform the low tension battery current to a current of sufficient voltage to jump the air gap at the plug. The current from the battery is first passed through the primary coil, which is composed of several layers of coarse wire wound around a core of lengths of soft iron wire to form an electro-magnet. Another coil composed of a large number of turns of finer wire is wound over the primary coil. When a current of electricity of low voltage passes through the primary coil, a current of high electro-motive force is delivered from the terminals of the secondarywinding.

The induction coil is usually provided with a device known as a “vibrator," which is an automatic circuit breaker actuated by the magnetism of the core. Every time the circuit is made and broken at the vibrator contact points an electrical impulse is induced at the secondary winding, and the more rapid the vibration, the greater the number of impulses in the secondary current. A simple coil, such as used for single cylinder ignition, is shown at Fig. 68. As will be seen, three terminals are provided on the coil exterior, one leading to the battery, one to the commutator, and the remaining one to the spark plug. The vibrator spring A carries one of the contact points and the knurled head adujsting screw B carries the other. The amplitude of vibration is varied by moving the adjusting screw B or by increasing or decreasing the strength

of the vibrator spring. An induction coil for use with a multiple cylinder engine is composed of a number of simple coils fastened together in a common case and having enough terminals to make the proper connections in the outer circuit, as will be described in proper sequence. When the vibrator is in operation it moves so rapidly that it makes a buzz like that of some fast flying insect and when properly adjusted it may even give a definite musical note. If the vibrator action is

FIG. 68.-Induction Coil Used in Connection With Batteries to Produce Electric Spark in Gas Engine Cylinder.

not rapid the amount of secondary current produced will not be as great nor will it have as high voltage because the current output is dependent not only upon the number of impulses flowing through the primary coil but also upon the rapidity of the contacts at the vibrator.

Producing Spark in Cylinders.-The two common

appliances for producing an electric spark in the combustion chamber are shown at Fig. 69. That at A is a low tension igniter plate having a movable electrode. One of the spark points is carried in a fixed member or anvil which is shown detached so the construction may be understood and which is installed in and insulated from the main body of the igniter plate. The movable member, which is actuated by a bell crank, carries the other contact point and is normally in contact with the anvil. When it is desired to produce

FIG. 69.-Devices for Producing Spark in Engine Cylinder. A-Low Tension Igniter Plate. B-High Tension Spark Plug.

a spark the hammer is pulled away from the anvil by some mechanical means, usually a tappet rod operated by a plunger and cam in just the same manner as a valve is lifted. At the instant of breaking contact a hot spark takes place between the points and this explodes the mixture. There are other forms of low tension igniters but the type described is the most