element K to the negative element L, until the pressures on both are equal, when the current will stop flowing and the cell will be discharged. If a valve C is placed in the pipe, connecting the two tanks, the flow of the water may be stopped. If a switch G is placed on the wire connecting the two elements the flow of electricity may be stopped. If a water wheel D is placed in the pipe the flowing water may be made to do work. If a small motor H is placed on the wire J the flowing electricity may be made to do work. In automobile use this work is done in charging the induction coil for ignition purposes, producing light, etc. The weight of the water in tank A gives a certain pressure. The similar pressure in the electric battery is measured in volts. The pressure of the water causes a quantity to flow through the pipe. The similar quantity of electricity that is forced over a wire from a battery is measured in amperes. Parts of Circuit. To retain water the tank walls must have a certain degree of strength which is determined entirely by the size of the tank and the height above ocean level. To enable objects to hold an electrical charge they must be surrounded by something that will retain it. Any substance which holds a charge upon its surface and does not permit it to flow thereby corresponding to the walls of water reservoirs is termed an insulator. Some substances conduct electricity, others resist its flow. If two water tanks were connected by a rod of metal the water could not flow from one to the other. The rod of metal must be hollow to permit the water to pass through it. Solid metal is a barrier or insulator that prevents the passage of water. A pipe is a conductor of water. If two electrically charged bodies are connected by a piece of wood, glass, rubber, dry cloth, paper or similar materials there will be no passage of electricity, but if a metal rod is substituted, a current will flow from the body of higher potential to the other. In this case the metal rod or wire is a conductor of electricity. All metals and substances such as acid, water and the various liquids (except oils) conduct electricity so well as to be termed "conductors" though it is harder for the electrical current to flow through some kinds of metal than it is for it to pass through others. Copper, aluminum and silver are very good elec trical conductors, steel or iron is next in order, while some alloys, such as German silver, offer considerable resistance to the flow of current. Materials such as wood, glass, rubber, etc., and air, conduct electricity so badly as to be termed insulators. What would normally be an insulator to a current of low potential may be ruptured by a current of higher potential or pressure which can break down the resistance. From the foregoing it will be evident that a current is produced by the passage of electricity from one body to another and that current can only flow through certain materials and that some substances act as a barrier to the current flow just as a valve stops the flow of water. With a valve in the water pipe, providing that the parts were sufficiently strong, closing the valve breaks the continuity of the pipe and stops the flow of water. The same is true of electricity, it must have a complete circuit or the currents cannot pass. An electrical circuit is said to be an open circuit when the current cannot flow and a closed circuit, if there is a continuous path for the electricity. A closed circuit therefore is one made up entirely of apparatus and wires capable of conducting electricity, including some form of generator of electrical energy which acts as a pump to produce a flow. The flow of current is from the electrical generator, through wires to the piece of apparatus to be operated and from that piece of apparatus back again to its source. If we connect the terminals of the battery through the wire to the bell, after energizing the bell magnets the electricity does work by ringing the bell. It flows from the positive or carbon terminal of the battery through the wire to the bell and after energizing the bell magnets, it returns through another conductor to the zinc or negative terminal of the battery. Inside of the cells, the flow is from the negative member to the positive member. Any closed circuit may be made an open. circuit by including an insulating body which resists current flow. This body is always of such a form that it can be temporarily bridged over by a conductor when it is desired that the current pass through the circuit. All electrical circuits must comprise a source of current, wires to carry it, a switch to interrupt it and apparatus to be actuated by it. A Current Production by Chemical Action.-The simplest method of current generation is by various forms of chemical current producers which may be either primary or secondary in character. A simple form of cell is shown in section at Fig. 3, A, and as the action of all devices of this character is based on the same principles it will be well to consider the method of producing electricity by the chemical action of a fluid upon a metal. The simple cell shown Fig. 2.-Diagrams Outlining How Current Voltage is Reduced by Increasing Resistance in Circuit. A-Water Flow Reduced by Shut-off Valve. B-Electric Flow Reduced by Rheostat, an Equivalent of the Valve in the Water System. consists of a container which is filled with an electrolyte which may be either an alkali or acid solution. Immersed in the liquid are two plates of metal, one being of copper, the other zinc. A wire is attached to each plate by means of suitable screw terminals. If the ends of the plates which are not immersed in the solution are joined together a chemical action will take place between the electrolyte and the zinc plate; in fact, any form of cell consists of dissimilar elements which are capable of conducting electricity im mersed in a liquid which will act on one of them more than the other. The chemical action of electrolyte on the zinc liberates gas bubbles which are charged with electricity and which deposit themselves on the copper plate. The copper element serves merely as a collecting member and is termed the "positive" plate, while the zinc which is acted upon by the solution is termed the "negative" member. The flow of current is from the zinc to the copper plate through A Fig. 3.-Simple Primary Cell Used to Produce Electric Current. the electrolyte and it is returned from the copper plate to the zine element by the wiring which comprises the external circuit. While in the cell shown zinc and copper are used, any other combination of metals between which there exists a difference in electrical condition when one of them is acted upon by a salt or acid may be employed. Any salt or acid solution will act as an electrolyte if it will combine chemically with one of the elements and if it does not at the same time offer too great a resistance to the passage of the electric current. The current strength will vary with the nature of the elements used, and will have a higher value when the chemical action is more pronounced between the negative member and the electrolyte. As the vibrations which obtain when the automobile is driven over highways makes it difficult to use cells in which there is a surplus of liquid, a form of cell has been devised in which the liquid electrolyte is replaced by a solid substance which cannot splash out of the container even if the cell is not carefully sealed. A current producer of this nature is depicted in section at Fig. 3, B. This is known as a dry cell and consists of a zinc can in the center of which a carbon rod is placed. The electrolyte is held close to the zinc or negative member by an absorbent lining of blotting paper, and the carbon rod is surrounded by some depolarizing material. The top of the cell is sealed with pitch to prevent loss of depolarizer. The depolarizer is needed that the cell may continue to generate current. When the circuit of a simple cell is completed the current generation is brisker than after the cell has been producing electricity for a time. While the cell has been in action the positive element becomes covered with bubbles of hydrogen gas, which is a poor conductor of electricity and tends to decrease the current output of the cell. To prevent these bubbles from interfering with current generation some means must be provided for disposing of the gas. In dry cells the hydrogen gas that causes polarization is combined with oxygen gas evolved by the depolarizing medium and the combination of these two gases produces water which does not interfere with the action of the cell. Carbon is used in a dry cell instead of copper because it is a cheaper material and the electrolyte is a mixture of salammoniac and chloride of zine which is held in intimate contact with the zinc shell which forms the negative element by the blotting paper lining. Wiring Dry Cells.-When dry cells are used for ignition there. are two practical methods of connecting these up. At least four dry cells are necessary to secure satisfactory ignition and much. more energetic explosions will be obtained if five or six are used. The common method is to join the cells together in series as shown at Fig. 4, A. When connecting in this manner the carbon terminal. of one battery is always coupled to the zine binding post of its |