cells will ignite small engines such as used in motorcycles, five or six will be needed on automobile engines employing high-tension ignition. When the make-and-break system, or low-tension method, is used eight or ten cells are necessary. If the engine is a multiple cylinder one, it will draw more current than a single cylinder type because of the greater frequency of sparks. On four-cylinder cars dry cells should be joined in multiple series, which is the most economical arrangement. Cells used in multiple connection are more enduring than if the same number were used independently in single-series connection. A disadvantage of a dry cell battery is that it is suited only for intermittent service and it will soon become exhausted if used where the current demands are severe. For this reason most automobiles in which batteries are used for ignition employ storage or secondary batteries to furnish the current regularly used and a set of dry cells is provided for use only in cases of emergency when the storage battery becomes exhausted.

Principles of Storage Battery Construction.-Some voltaic couples are reversible, i. e., they may be recharged when they have become exhausted by passing a current of electricity through them in a direction opposite to that in which the current flows on discharge. Such batteries are known as "accumulators" or "storage batteries." A storage battery belies its name as it does not store current and its action is somewhat similar to that of the simpler chemical cell previously described. In its simplest form a storage cell would consist of two elements and an electrolyte, as outlined at Fig. 5, A. The storage battery differs from the primary cell in that the elements are composed of the same metal before charging takes place, usually lead instead of being zinc or carbon. One of the plates is termed the "positive" and may be distinguished from the other because it is brown, or chocolate in color after charging, while the negative plate is usually a light gray of leaden color. The active material of a charged storage battery is not metallic lead but oxides of that material.

The simple form shown at A consists of two plates of lead which are rolled together separated by insulating bands of rubber at the top and bottom to keep them from touching. This roll is immersed in an electrolyte composed of a weak solution of sul

phuric acid in water. Before such a cell can be used it must be charged, which consists of passing a current of electricity through it until the lead plates have changed their nature. After the charging process is complete the lead plates have become so changed in nature that they may be considered as different substances and a chemical action results between the negative plate and the electrolyte and produces current just as in the simple cell

Fig. 5.-Types of Accumulators or Storage Batteries. A-Simple Form of Cell. B-Battery Composed of Three Cells Such as Commonly Used for Ignition Purposes.

shown at Fig. 3, A. When the cell is exhausted the plates return to their metallic condition and are practically the same, and as there is but little difference in electrical condition existing between them, they do not deliver any current until electricity has been passed through the cell so as to change the lead plates to oxides of lead instead of metallic lead.

When storage cells are to be used in automobile work they are combined in a single containing member, as shown at Fig. 5, B,

which is a part sectional view of a Geiszler storage battery. The main containing member, a jar of hard rubber, is divided into three parts. Each of these compartments serves to hold the elements comprising one cell. The positive and negative plates are spaced apart by wood and hard rubber separators which prevent short circuiting between the plates. After the elements have been put in place in the compartments forming the individual cells of the battery, the top of the jar is sealed by pouring a compound of pitch and rosin, or asphaltum, over plates of hard rubber, which keeps the sealing material from running into the cells and on the plates. Vents are provided over each cell through which gases produced by charging or discharging are allowed to escape. These are so formed that while free passage of gas is provided for, it is not possible for the electrolyte to splash out when the vehicle is in motion.

It will be evident that this method of sealing would not be practical on a cell where the members attacked by the acid had to be replaced from time to time, but in a storage battery only the electrolyte need be renewed. When the plates are discharged they are regenerated by passing a current of electricity through them. New electrolyte or distilled water can be easily inserted through holes in which the vents are screwed. The cells of which a storage battery is composed are joined together at the factory with bars of lead which are burned in place and only two free terminals are provided by which the battery is coupled to the outer circuit.

The capacity of a storage battery depends upon the size and the number of plates per cell, while the potential or voltage is determined by the number of cells joined in series to form the battery. Each cell has a difference of potential of two and two tenths volts when fully charged, therefore a two-cell battery will deliver a current of four and four tenths volts and a three-cell type, as shown in part section at Fig. 6, will give about six and six tenths volts between the terminals. In the form shown each cell is composed of a number of plates and their separators. One group of the plates is positive, the remaining negative members. The size of storage battery to be used depends upon the number of cylinders of the engine and also if battery is to be used for

starting and lighting purposes as well as ignition. Four-cylinder motors usually take a six-volt, sixty-ampere-hour battery, but it is desirable to supply a six-volt battery having eighty-ampere-hour capacity for six-cylinder motors for ignition only. For lighting or starting 100 ampere hour batteries are needed.

When chemical current producers are depended upon to supply the electricity used for ignition, two distinct sets are provided,

Fig. 6.—Special Storage Battery Designed to Furnish Lighting and

Starting Current.

one for regular service and the other for emergency use in event of failure of that which is depended upon regularly. The common practice is to provide an accumulator or storage battery for normal use and a set of dry cells, which are cheaper in first cost and which do not deteriorate if not used for some time, for emergency service. When two sources of current are thus provided, a switch is included in the circuit so that either set may be used at will. The zinc terminal of the dry battery and the negative terminal of the storage battery are joined together by a suitable

conductor and are grounded by running the wire attached to them to some metal part of the chassis such as the crank case or frame side member. The remaining terminals, which are the positive of the storage battery and the carbon of the dry cell, are coupled to distinct terminals on the switch block.

The fact that any battery cannot maintain a constant supply of electricity has militated against their use to a certain extent and the modern motorist demands some form of mechanical generator driven from the power plant, which will deliver an unfailing supply of electricity and keep the battery charged. The strength of batteries is reduced according to the amount of service they give. The more they are used the weaker they become. The modern multiple cylinder engines are especially severe in their requirements upon the current producer and the rapid sequence of explosions in the average six- or eight-cylinder motor produce practically a steady drain upon the battery. When dry cells are used their discharge rate is very low and as they are designed only for intermittent work, when the conditions are such that a constant flow of current is required, they are unsuitable and will soon deteriorate. A more comprehensive discussion on the care, repair and charging of storage batteries will be found in the following chapter.

Fundamentals of Magnetism Outlined. To properly understand the phenomena and forces involved in the generation of electrical energy by mechanical means it is necessary to become familiar with some of the elementary principles of magnetism and its relation to electricity. The following matter can be read with profit by those who are not familiar with the subject. Most persons know that magnetism exists in certain substances, but many are not able to grasp the terms used in describing the operation of various electrical devices because of not possessing a knowledge of the basic facts upon which the action of such apparatus is based.

Magnetism is a property possessed by certain substances and is manifested by the ability to attract and repel other materials susceptible to its effects. When this phenomena is manifested by a conductor or wire through which a current of electricity is flowing it is termed "electro-magnetism." Magnetism and electricity are