it is capable of doing work. The passage of electricity through any piece of apparatus is termed a current. If the flowing of the electrical charges is continuous it is called a direct current. If the charges are not continuous but flow always in the same direction it is termed a "pulsating" current. If an electrical charge flowing in one direction is followed by another charge flowing in the opposite direction, an "alternating" current is produced.

It will be evident that to obtain a regular flow a constant supply of electricity, such as afforded by some electrical generator is required. The simplest analogy to permit the reader to understand the passage of a current is the flow of a stream of water. A number of comparisons can be made between water and the electric current which tend to simplify the explanation, though it is understood that there can be little in common between such a tangible fluid as water is and electricity which is intangible and only considered a fluid for convenience. To form some conception of this force, it is well to consider that we are able to place various bodies in different electrical relations. A stick of sealing wax or a hard rubber comb, rubbed on a coat sleeve, will attract bits of paper, feathers and other light objects. The sealing wax or rubber is said to be charged with electricity which has been produced by friction against the coat sleeve. Any body charged with electricity may be considered one whose surface is supplied with either an overcharge or undercharge of electricity. The overcharged body always tends to discharge to the undercharged body in order to equalize a difference in pressure existing between them. An electrical machine capable of producing current may distribute this current as desired, providing the current is sufficiently strong to overcome the resistance to its motion of the parts comprising the external circuit.

Why Current Flows.-The action of an electrical machine in regulating the distribution of electricity may be considered to be the same as that of a pump which takes water from one tank and supplies it to another at a higher level. If for these reservoirs we consider bodies insulated from each other, we can, with an electrical generator take electricity from one that has been overcharged and supply it to another which is undercharged. If we had two

tanks of water at the same level, one container being full and the other nearly empty, merely connecting these with the pipe in which a valve was placed would permit the water from the full reservoir to pass into the nearly empty one, till both contained equal amounts of liquid. As is the case with tanks of water if two bodies are charged with unequal amounts of electricity the electrical charge will tend to equalize itself when the two bodies are connected together with a conductor that will allow the passage of electricity. Any time the flow of water from one tank to the other is to be interrupted, up to that point when equilibrium is reached, closing the valve will obstruct the conductor and shut off the flow of water. It is possible to utilize a switch in the electrical conductor, which will do the same thing as the valve does in the water pipe. Water cannot flow through the closed valve because of the resistance the valve offers. If the valve is weak and the water pressure is sufficiently high it is possible for the water to burst the restraining walls in the valve and continue to flow. It is evident therefore that the strength of the valve parts must be proportioned according to the pressure of the water stream. A switch interposed in an electrical conductor will, when opened, leave an air gap in this conductor that offers so much resistance to the flow of current that the electricity cannot pass. Closing the switch so that the continuity of the conductor is re-established will enable the current to flow. An electrical circuit is different from a water circuit, inasmuch as electricity must always return to its source. The greater the difference in the quantities of the electrical charge the greater the tendency to reach the state of equilibrium. This difference in electrical conditions or amount of electrical charge is termed "difference of potential," and high or low potential or "electro motive force' in any electrical system indicates a large or small difference of charge or electrical condition at different parts. Just as in the case of the tanks filled with different amounts of water, and in which as a result there is a difference of level, the flow is always through a conductor from the point of higher to that of lesser potential. If we had a tank of water ten feet from the ground the water would flow faster through a certain size hole than if the tank were but two feet from the ground. Not only would the tank be

emptied quicker but the water would have a greater head or pressure. The same condition exists on electrified bodies as the greater the difference of potential or level between them the more rapid the flow and the greater the pressure of the current.

The levels of liquids in the tanks instead of being compared to each other might be referred to that of an ocean of constant level. Water might be pumped into the ocean from one or from the ocean to one or both so as to affect the level of water in the tanks with respect to the larger quantity in the ocean of constant height. Electricity can be considered in the same manner. It can be taken

Fig. 1.—Diagrams Illustrating How Current Pressure Causes Electricity to Flow by Comparing It to a Flow of Water from One Tank to Another.

from an ocean of electricity, which may be represented by the electrical charge present at all points of the earth or the earth can be used, as it invariably is, as a receptacle for the charges obtained from electrical producers.

In Fig. 1 is shown two tanks, A and B, connected by a pipe. Let tank A, which is filled with water, represent the positive element K of the cell F, and the empty tank B the negative element L. Let pipe E connecting the two tanks represent wires J connecting the two elements. It is evident that water will flow through the pipe from the full tank to the empty tank until both contain the same quantity and the pressures are equal. Likewise in the battery cell electricity will flow through the wire from the positive

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.