ELECTRICITY

the name given to the ultimate cause of electrical phenomena. The laws governing these phenomena are well known, but the actual nature of electricity has not yet been fully revealed, although much light has been thrown on the subject by recent researches. (See ELECTRON.) Although the practical applications of electrical phenomena have all been developed within the last fifty years, the production of an electric charge by friction, as demonstrated by the power of rubbed amber | th to attract light bodies, was observed by a Greek philosopher as long ago as 600 B.c. The Greek name for amber, ndextpov (electron), is the root from which our word electricity is derived. Friction was the only artificial source of electricity known until Galvani, near the close of the eighteenth century, accidentally obtained it by the contact of two metals with the limbs of a frog; and Volta, developing Galvani’s discovery, invented the first galvanic or voltaic battery. The discovery by Faraday in 1831 of the principle of the production of an electromotive force by the motion of a conductor in a magnetic field, laid the foundation for the development of the electric generator (q.v.), and thus of modern electric power supply. The study of electrical phenomena is conveniently divided into two branches, one dealing with stationary charges of electricity (clectro-statics), the other with electric currents (current electricity). Electrostatics. Ifa pair of ebonite rods be electrified by friction with flannel, then by suspending the one rod and presenting the other to it, it is easily demonstrated that a mutual mechanical force of repulsion exists between them. If now a gla rod be electrified by friction with silk, it will be found that it attracts the suspended electrified ebonite rod. These experiments reveal the facts that electric charges may be of two opposite kinds, and that like charges repel one another, while unlike charges attract one another. The charge produced on gla by friction with silk is called positive $ that produced on ebonite by friction with flannel is called negative. The kind of charge produced depends not merely on the material rubbed, but also on the material of the rubber. Thus a warm dry gla rod becomes negatively electrified when rubbed with fur. The rubber always becomes electrified with a charge of the opposite kind to that produced on the material rubbed, and these two charges are equal in amount. All bodies may be electrified by friction, but those which allow a free moyement of the charge over them (such bodies are called conductors, to distinguish them from insulators, which do not allow this free movement) must be held by an insulating handle, or d else the charge will be removed any quickly as it is produced. Coulomb proved.that the magnitude of the mutual ‘mechanical, fone exerted between two jounts J depends on the amounts charges and the distance poe em. Faraday called attention to the influence of the medium jn! which the charges are placed. Thus if two charges of gq; and q. units respectively are placed aan centiL.aetres. apart in a given medium, the mechanical force a in dynes exerted between them is M95 Ka? provided the dimensions of the bodies on which the charges are concentrated given by the equation f= 100 ELECTRICITY are small in comparison with d. The coefficient K is called the dielectric const ané of the medium, and its value is taken as unity for air. In accordance with this relationship, unit charge is defined as that charge which repels an equal and similar charge placed at a distance of 1 centimetre in air, with a force of 1 dyne. if the medium surrounding a charged body be explored with a unit charge, a mechanical force varying in magnitude and direction from point to point will be touna to act on the unit charge. Im such a case, an electric field is said to exist in the medium. The strength of the electric field at any point is defined as numerically equal to the mechanical force which would act on a unit charge placed in air (or more strictly in a vacuum) at that point. The direction of the electric field at any point is defined to be the direction of the mechanical force acting on a unit positive charge placed at that point. It should be noted that the strength of the electric field and the mechanical force are numerically equal only when the dielectric constant of the medium is unity. Thus if F is the field strength, K.the dielectric constant, and U the mechanical force acting on a unit charge, F = KU. ‘It is very convenient to represent an ‘electric field by means of what are *called lines of electric force. If lines are drawn, starting from a positive charge and ending on a negative charge, such that the tangent to the line at any point is the direction of the electric force at that point, these lines are called lines of electric force. They can be drawn in such a way that the strength of. the electric field at any point is numerically equal to the number of lines of electric force pa ing through unit area surrounding that point (and taken at right angles D * to the direction of the force). The lines of electric force will thus completely represent the electric field. Further, if the following properties are attributed to the lines of electric force, viz. (a) that a line of electric force tends to shorten itself as far as po ible; (6) that lines of electric force mutually repel one another; then ali the phenomena due to the presence of an electric field may be interpreted by the behaviour of the lines of electric force Figs. 1 and 2 show the lines of electric force in the space surrounding two charged spheres. Fig. 1 shows the case where the charges are opposite, fig. 2 the case where they are similar. In fig. 1 the attraction between the spheres may be thought of as due to the tendency of the lines of force to shorten themselves. Similarly, the mutual repulsion of the spheres in fig. 2 may be regarded as a consequence of the mutual repwsion of the lines of force. The distribution of a charge upon an insulated conductor isoJated in space depends upon the shape of the conductor. Jf the conductor is the charge is uniformiy spherical, distributed. If the curvature varies from point to point, the quantity of charge per unit area, or the electric surface densiiy, will vary from point to point. The sharper the curvature is, the greater the surface density will 101 ELECTRICITY be. In fig. 3 the distance of the dotted lines from the surface of the conductors is proportional to the surface density. These lines, therefore, give a graphical representation of the distribution of charge, In sharply pointed conductors nearly the whole charge will be concentrated at the pointed end. Owing to the large charge per unit area at the pointed part, particles of dust, water-vapour, ., will be powerfully attracted, will become charged by conduction, and will then be powerfully repelled. In this way the original charge will be rapidly di ipated. This effect may be shown by keeping a _ sharply pointed conductor powerfully charged by an electric machine. The streaming of the particles from the point produces a wind which is sufficient to blow out the fame of a candle. Conductors which are intended to retain their charge for a long period must be smooth and polished, and the maximum curvature must be as small as po ible. In lightning-conductors practical advantage is taken of this ‘power of points’ to di ipate a charge rapidly. The distribution of the charge on a conductor is influenced by the presence of other conductors, whether charged or not. This is due to what is called electrostatic induction. If an uncharged insulated conductor B is brought near a charged conductor A, a charge of the opposite kind is induced on the parts of B nearer to A, and a charge of the same kind on the parts farther away from A. Since was originally uncharged, these induced charges are equal in amount. If B is now removed to a distance, the induced charges neutralize one another, and B returns to its original uncharged state. While B is near A, let the induced charge of the same kind as the charge on A be neutralized by touching B with an earth-connected conductor, say the finger. On removing B to a distance, it will no longer be uncharged as before, but will have a charge of the opposite kind from that on A. B is now said to have been charged by induction. It is instructive to view these phenomena in the light of the conception of lines of electric force. When B is brought up towards A, some of the lines of force a ociated with the charge on A, and originally linked to surrounding objects, will now, owing to the tendeney of the lines to shorten themselves, be linked to B. At the same time an equal number of lines (of opposite direction relative to B) will link B to the nearest surrounding | objects. starts from a positive charge and ends on a negative charge, the charge on the parts of B nearer to A will be of the opposite kind to that on A, but the charge on the part farther from A will be of the same kind as that on A. When the earth-connected conductor is brought near B, the lines formerly linking B to surrounding objects will link B to the earth connected conductor. Finally, when the latter touches B, these lines shorten themselves indefinitely and disappear. The attraction of light particles to a charged body is explained by electrostatic induction. The charge of opposite kind induced on the particle being nearer than that of the same kind, the particle is attracted. When it touches the charged body, the charge of opposite kind is neutralized, Fig. 4. Induction and Lines (or tubes) of Force and the charge of like kind now left on the particle causes repulsion to take place. If the electrified body is an insulator, the neutralization of the charges only takes place slowly, and consequently it may be some time before the particle is repelled. If two charged conductors be connected by a wire, in general it will be found that a flow of electricity from one to the other will take place. This flow is said to be due to a difference of electric potential between the two conductors. If no flow takes place, then the difference of potential is zero. Electric potential difference (the contraction P.D. is commonly used) is numerically equal to the work done in carrying a unit charge from the one conductor to the other. If the work is done against the electric forces, in moving a unit positive charge from A to B, then B is said to be at a higher potential than A. Although actually it is with differences of potential that we have always to deal, it is convenient in many casen to refer these differences to a zero, and speak of the potential at a point. The ideal zero of potential would be the potential at a point infinitely far removed from all electrified bodies. Since by definition a line of force) In practice it is convenient to regard 102 ELECTRICITY 99 ELECTRICITY the potential of the earth as zero. | The simplest appliance of this kind is The potential at a point is then, the electrophorus, which consists of a numerically equal to the work done} disc of ebonite or other suitable in carrying a unit positive charge | material with a metallic base,,and a from earth to the point. The notential at every point on a conductor is obviously the same, for if it were not so, a flow of charge would take place and equalize the potential. If an insulated uncharged conductor be connected by a wire to a charged conductor, a flow ef charge will take place until every point on both conductors is at the same potential. The quantity of charge which each conductor will then have depends on what is called the capacity of the conductor. The capacity of a conductor is defined as the quantity of electricity with which it must be charged in order to raise its potential from zero to unity. Thus if Q be the quantity, ¥V the potential, and C the capacity, The potential of a conductor is, therefore, directly propertional to the charge upon the conductor, and inversely preportional to the capacity of the conductor. The capacity of a conductor may be increased by placing close to it another conductor which is kept at zero potential. Such an arrangement is called a condenser. The Leyden jar (see LEYDEN JAR) is a well-known example of a condenser. The capacity depends not merely on the dimensions of the conductors and the distance between them, but also upon the nature of the dielectric separating them. The ratio of the capacity of a condenser with a given diclectric to the capacity it would have with an air dielectric, is called the specific inductive capacity of the dielectric. Numerically the specific inductive capacity of a dielectric is we have C= —. V Fig. 5. Electrophorus equal to the dielectric constant already mentioned. In the experimental investigation of electrostatic phenomena it is convenient to have appliances which will supply charges as they are required. metal disc of slightly smaller diameter Q merrs{ Fig.6. Wimshurst Machine having an insulating handle attached at right angles to its surface (see fig. 5). To use the electrophorus, the ebonite is given a negative charge by striking it with fur or flannel. The metal disc is then placed on top of the ebonite plate. Since the ebonite is an insulator, no general neutralization of the positive induced charge on the lower side of the metal disc can take place. The negative charge on the upper surface of the metal disc is then neutralized by touching with the finger. The disc is thus left positively charged. The disc is then lifted by the insulating bandle, and the charge utilized as required. Theoretically speaking, this proce may be repeated continuously without affecting the original charge on the ebonite plate, but in practice the ebonite has to be re-excited from time to time on account of the lo of charge by leakage. More elaborate appliances of many different forms have been used, but the only one of these electric machines, as they are called, which is now commonly employed is the Wimshurst machine. This machine consists of two circular plates of gla 103 ELECTRICITY or ebonite ces equal even numbers of tin-foil sectors symmetrically placed on their outer surfaces. A pain of bra arms carrying wire brushes, which simultaneously make contact with diametrically opposite sectors on each plate, is so arranged as to lie at an angle of about 45° to the horizontal,and to be at right angles to one another. A pair of combs is placed at each end of the horizontal] diameter of the plates, so that the sectors pa close to the teeth of these combs. The combs serve as collectors, and are connected one pair to the positive pole, and the other pair to the negative pole of the machine. The general appearance of the machine is shown in fig. 6. The machine acts on the induction Fig. 7. Dlectroscope principle, and if kept warm and dry is self-exciting. The electroscope is a simple piece of apparatus for detecting the presence of an electric charge, determining its sign (positive or negative), and making a very rough comparative ‘estimate of its potential. It consists of a pair of strips of gold-leaf attached to a bra rod terminating in a bra cap. The whole is enclosed in a gla case, or a case having gla sides. The base is made of conducting material. The sides of the case are coated internally with tinfoil (or two rods connected to the base project upwards to the level of the gold-leaf strips). The general appearance of one form of electroscope is shown in fig. 7. The gold-leaf strips, the bra rod, and the cap must be carefully insulated. When a charged body is brought near the electroscope the leaves become charged similarly by induction. The repulsion due to the similar charges causes the leaves to diverge. ’ If the cap be touched with the finger, the charge on the leaves is neutralized, and the leaves collapse. On removing the charged body the leaves diverge again, owing to the spreading of the charge on the cap, which was held by the inducing charge, over the whole conductor, including the leaves. The electroscope is thus charged by induction. lt may also be charged by conduction, i.e. by the direct transfer of a charge to the electroscope. When we know the kind of charge, positive or negative, which has been given to the electroscope, an unknown charge can be tested. If the approach of the unknown charge causes a further divergence of the leaves, then it is of the same kind as that with which the electroscope is charged. When accurate quantitative measurements have to be made, an instrument called an electrometer is used. This instrument, the development of which is due chiefly to Lord Kelvin, is capable of making accurate measurements of electrostatic potential differences down to quite low values. E entially an electrometer copsists of a light suspended conductor which moves within four fixed quadrants. Opposite pairs of these quadrants are connected together, one pair to one terminal, and the other pair to the other terminal of the instrument. The P.D. to be measured is applied at these terminals. The suspended conductor or ‘needle’ is charged to a definite high potential, and the deflection produced is observed from the movement of a spot of light reflected from a mirror attached to the suspending fibre. In this case the deflection is proportional to the P.D. between the quadrants. For measuring a high P.D., the needle may be connected to one pair of quadrants. With such an arrangement the instrument is le sensitive, and the deflection is proportional to the square of the P.D. between the quadrants. Current Electricity. The phenomena connected with the fiow of electricity through a conductor come under this heading. Such a flow of electricity will take place if by some means the ends of the conductor are maintained at different potentials. An electric current is then said to exist in the conductor. The difference of potential may be maintained by chemical action (see DANIBLL’S CELL; ELECTRIGO BATTERY), by electrodynamic action (see GENERATOR), or by heat action (see THERMO-ELEOTRICITY). The magnitude of the 104 ELECTRICITY current which will flow when a steady P.D. is maintained between the ends of the conductor is determined by what is called the electrical resistance of the conductor. The resistance R is defined as the ratio of the applied potential difference V to the Vv = —. This I is a partial expre ion of Ohm’s Law for the Electric Circuit, which in its most general form states that the current which flows at any instant in an electric circuit is equal to the algebraic sum of the electromotive forces existing in the circuit at that instant, divided by the total resistance in the circuit at that instant (see ELECTROMOTIVE FORCE). For the particular case where the algebraic sum _ E of the electromotive forces is steady and the total res is current I produced, i.e. tance R is not varying, we have here This is the form which applies to steady direct currents. If the current is changing (whether alternating or merely varying in value), varying E.M.F.’s, in addition to the applied E.M.F., exist in the circuit, and the eae expre ion no longer holds ood. The resistance of a conductor depends on its material, and varies directly as the length, and inversely as the cro -section of the conductor. Z Thus R = re where p is the specific resistance of the material, J the length of the conductor, and A the cro sectional area of the conductor. The specific resistance is the resistance between opposite faces of a unit cube of the material at a definite temperature (usually 0° C.). The resistance of a conductor varies to a greater or pase extent with variation of tempera ure. For pure metals the resistance increases considerably with increase of temperature. With certain alloys the change is so slight as to be negligible. In some alloys, and in carbon and insulating materials, the resistance falls with increase of temperature. Measurement of Resistance. Low resistances can most conveniently be measured by a fall of potential method, based on the relationship R.=—. ny by an ammeter, and the potential difference by a low-reading voltmeter (see ELECTRICAL MEASURING INSTRUMENTS). Where greater accuracy is required, a constant current is sent through the resistance to be measured, The current may be read and also through a known standard resistance of about the same value. A sensitive galvanometer (see GALVANOMETER) is used to compare tha P.D. acro the unknown resistance with that a->o the standard. Since the current is the same through both, the resistances will be proportional to the galvanometer deflections, and from the known value of the standard resistance the value of the unknown resistance can be calculated. kesistances of moderate value are best measured by a Wheat stone Bridge, or one of its modifications (see WHEAT STONE BRIDGE). A substitution method is more suitable for high resistances. IMG:fig103_1.jpg: IMG:fig104_1.jpg: IMG:fig106_1.jpg: