THE ELECTRIC CURRENT

112.] Let A and B be two metal bodies connected respectively with the inner and outer coatings of a Leyden jar, the inner coating of the jar being positive, so that the potential of A is higher than that of B. Let C be a gilt pith ball suspended by a silk thread. If C is brought into contact with A and B alternately, it will receive a small charge of positive electricity from A every time it touches it, and will communicate positive electricity to B when it touches B.

Fig. 28.

There will thus be a transference of positive elec- tricity from A to B along the path travelled over by the pith ball, and this is what occurs in every elec- tric current, namely, the passage of electricity along a definite direction. During the motion of the pith ball from A to B it is charged positively, and the electric force between A and B tends to move it in the direction from A to B. After touching B, it becomes charged negatively, so that the electric force, during its return journey, acts from B to A. Hence the ball is acted on by the electric force always in the direction in which it is moving at the time, so that if it is properly suspended the electric force will not only keep up the backward and forward motion, but will communicate to the moving ball an amount of energy which it will expend in a series of rattling blows against the balls A and B. The current of positive electricity from A to B is thus kept up by means of the electromotive force from A to B.

113.] The phenomenon we have been describing may be called a current of Convection. The motion of the electrification takes place in virtue of the motion of the electrified body which conveys or carries the electricity as it moves from one place to another. But if, instead of the pith ball, we take a metal wire carried by an insulating handle, and cause the two ends of the wire to touch A and B respectively, there will be a transference of electricity

CONVECTION AND CONDUCTION CURRENTS from A to B along the wire, though the wire itself does not move. What takes place in the wire is called a current of Conduction. The effects of the current of conduction on the electrical state of A and B are of precisely the same kind as those of the current of convection. In both cases there is a transference of electrification from one place to another along a continuous path.

In the case of the convection of the charge on the pith ball we may observe the actual motion of the ball, and therefore in this case we may distinguish between the act of carrying a positive charge from A to B and that of carrying a negative charge from B to A, though the Electrical effects of these two operations are identical. We may also distinguish between the act of carrying a number of small charges from A to B in rapid succession and with great velocity, and the act of carrying a single great charge, equivalent to the sum of these charges, slowly from A to B in the time occupied by the whole series of journeys in the other case.

But in the case of the current conduction through a wire we have no rea- son to suppose that the mode of transference of the charge resembles one of these methods rather than another. All that we know is that a charge of so much electricity is conveyed from A to B in a certain time, but whether this is effected by carrying positive electricity from A to B, or by carrying negative electricity from B to A, or by a combination of both processes, is a question which we have no means of determining. We are equally unable to determine whether the ‘velocity of electricity’ in the wire is great or small. If there be a substance pervading bodies, the motion of which constitutes an electric current, then the excess of this substance in connexion with a body above a certain normal value constitutes the observed charge of that body, and is a quantity capable of measurement. But we have no means of estimat- ing the normal charge itself. The only evidence we possess is deduced from experiments on the quantity of electricity evolved during the decomposition of one grain of an electrolyte, and this quantity is enormous when compared with any positive or negative charge which we can accumulate within the space occupied by the electrolyte. If, then, the normal charge of a portion of the wire the millionth of an inch in length is equal to the total charge trans- ferred from A to B, the transference may be effected by the displacement ofMEASURE OF CURRENT.

the electricity in the wire whose linear extent is only the millionth of an inch. It is therefore quite possible that the velocity of electricity in a telegraph wire may be exceedingly small, less, say, than the hundredth of an inch in an hour, though signals, that is to say, changes in the state of the current, may be propagated along the wire many thousands of miles in a second. Since, therefore, we are ignorant of the true linear velocity of an electric current, we must measure the strength of the current by the quantity of elec- tricity discharged through any section of the conductor in the unit of time, just as engineers measure the discharge of water and gas through pipes, not by the velocity of the water or gas, but by the quantity which passes in a minute.

114.] In many cases we have to consider the whole quantity of electricity which passes rather than the rate at which it passes. This is especially the case when the current lasts only a very short time, or when the current is considered merely as a transition from one permanent state of the system to another. In these cases it is convenient to speak of the total current as the Electric Displacement, the word displacement indicating the final result of a motion without reference to the rate at which it takes place. The passage of a given quantity of electricity along a given path is called an Electric Discharge. Classification of bodies according to their relation to the transference of electricity.

115.] For the sake of distinction we shall consider a portion of matter whose ends are formed by two equipotential surfaces having different poten- tials, and whose sides are formed by lines of electric current or displacement. The ends of the body are called its Electrodes, that at which electricity enters is called the Anode, and that at which it leaves the body is called the Cathode.

The excess of the potential of the anode over that of the cathode is called the External Electromotive Force. The Form of the body may vary from that of a long wire surrounded by air or other insulating matter to that of a thin sheet of the substance, the electricity passing through the thickness of the sheet.OHM’S LAW.

Bodies may be divided into three great classes according to the mode in which they are acted on by electromotive force,—Metals, Electrolytes, and Dielectrics.