Зображення сторінки





Nature of Electricity-Why Current Flows—Parts of Circuit-Conductors

and Insulators—Methods of Producing Electricity—How Primary Battery Generates Current-Wiring Dry Cells—How Storage Battery Works-Magnetism-Current Production by Induction-Magneto Action-Low Tension Magneto-High Tension Systems—Dynamo and Motor ActionMethods of Winding-Electrical Terms Defined—Electrical Measuring Instruments.

MANY forces exist in the universe the character of which have never been solved and may never be to the end of the world. We know these forces exist because their presence is made known by well understood phenomena. Among these forces gravitation, light, electricity and magnetism are prominent and even in this advanced age no one has a very clear conception of the nature of any of these forces nor would a presentation of theory and surmise be of any material benefit to those who are more concerned with the practical utility and the way these forces can be made of value to man than with a scientific presentation of theoretical causes.

Nature of Electricity.-A knowledge of electricity is of great value in permitting the reader to grasp clearly the principles underlying the operation of the various units comprising the ignition, starting and lighting systems of the modern motor car. If the following explanations are carefully studied it will be possible for one without any previous electrical training to understand intelligently the functions of electrical appliances and make it easier to locate and remedy troubles that are apt to materialize in these appliances. Electricity is a form of energy and is known because 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 pres

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

[blocks in formation]

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

« НазадПродовжити »