THE ELECTRIC FIELD

42. When an electrified body is enclosed in a conducting vessel, the total electrification of the interior surface of the surrounding vessel is invariably equal in numerical value but opposite in kind to that of the body.

This is true, however large this vessel may be.

It may be a room of any size having its floor, walls and ceiling of conducting matter.

Its boundaries may be removed further, and may consist of the surface of the earth, of the branches of trees, of clouds, perhaps of the extreme limits of the atmosphere or of the universe.

In every case, wherever we find an electrified insulated body, we are sure to find at the boundaries of the insulating medium, wherever they may be, an equal amount of electrification of the opposite kind. This correspondence of properties between the two limits of the insulating medium leads us to examine the state of this medium itself, in order to discover the reason why the electrification at its inner and outer boundaries should be thus related.

In thus directing our attention to the state of the insu- lating medium, rather than confining it to the inner conductor and the outer surface, we are following the path which led Faraday to many of his electrical discoveries.

43.] To render our conceptions more definite, we shall begin by supposing a conducting body electrified positively and insulated within a hollow con- ducting vessel. The space between the body and the vessel is filled with air or some other insulating medium. We call it an insulating medium when we regard it simply as retaining the charge on the surface of the electrified body. When we consider it as taking an important part in the manifestation of electric phenomena we shall use Faraday’s expression, and call it a dielectric medium. Finally, when we contemplate the region occupied by the medium as being a part of space in which electric phenomena may be observed, we shall call this region the Electric Field. By using this last expression we are not obliged to figure to ourselves the mode in which the dielectric medium takes part in the phenomena. If we afterwards wish to form a theory of the action of the medium, we may find the term dielectric useful.

Exploration of the Electric Field. Experiment XI.

(α) Exploration by means of a small electrified body.

44.] Electrify a small round body, a gilt pith ball, for example, and carry it by means of a white silk thread into any part of the field. If the ball is suspended in such a way that the tension of the string exactly balances the weight of the ball, then when the ball is placed in the electric field it will move under the action of a new force developed by the action of the electri- fied ball on the electric condition of the field. This new force tends to move the ball in a certain direction, which is called the direction of the electromo- tive force.

If the charge of the ball is varied, the force is sensibly proportional to the charge, provided this charge is not sufficient to produce a sensible disturbance of the state of electrification of the system. If the charge is positive, the force which acts on the ball is, on the whole, from the positively electrified body, and towards the negatively electrified walls of the room. If the charge is negative, the force acts in the opposite direction. Since, therefore, the force which acts on the ball depends partly on the charge of the ball and partly on its position and on the electrification of the system, it is convenient to regard this force as the product of two factors, one being the charge of the ball, and the other the electromotive force at that point of the field which is occupied by the centre of the ball. This electromotive force at the point is definite in magnitude and direction. A positively charged body placed there tends to move in the positive direction of the line representing the force. A negatively charged body tends to move in the opposite direction.

Experiment XII.

( β) Exploration by means of two disks. 45.] But the electromotive force not only tends to move electrified bodies, it also tends to transfer electrification from one part of a body to another.

Take two small equal thin metal disks, fastened to handles of shellac or ebonite; discharge them and place them face to face in the electric field, with their planes perpendicular to the direction of the electromotive force. Bring them into contact, then separate them and remove them, and test first one and then Fig. 15.

the other by introducing them into the hollow vessel of Experiment VII. It will be found that each is charged, and that if the electromotive force acts in the direction AB, the disk on the side of A will be charged negatively, and that on the side of B pos- itively, the numerical values of these charges being equal. This shews that there has been an actual transference of electricity from the one disk to the other, the direction of this transference being that of the electromotive force.

This experiment with two disks affords a much more convenient method of measuring the electromotive force at a point than the experiment with the charged pith ball. The measurement of small forces is always a difficult oper- ation, and becomes almost impossible when the weight of the body acted on forms a disturbing force and has to be got rid of by the adjustment of coun- terpoises. The measurement of the charges of the disks, on the other hand, is much more simple.

The two disks, when in contact, may be regarded as forming a single disk, and the fact that when separated they are found to have received equal and opposite charges, shews that while the disks were in contact there was a dis- tribution of electrification between them, the electrification of each disk being opposite to that of the body next to it, whether the insulated body, which is charged positively, or the inner surface of the surrounding vessel, which is charged negatively.

Electric Tension.

46.] The two disks, after being brought into contact, tend to separate from each other, and to approach the oppositely electrified surfaces to which they are opposed. The force with which they tend to separate is proportional to the area of the disks, and it increases as the electromotive force increases, not, however, in the simple ratio of that force, but in the ratio of the square of the electromotive force.

The electrification of each disk is proportional to the electromotive force, and the mechanical force on the disk is proportional to its electrification and the electromotive force conjointly, that is, to the square of the electromotive force.

This force may be accounted for if we suppose that at every point of the dielectric at which electromotive force exists there is a tension, like the ten- sion of a stretched rope, acting in the direction of the electromotive force, this tension being proportional to the square of the electromotive force at the point. This tension acts only on the outer side of each disk, and not on the side which is turned towards the other disk, for in the space between the disks there is no electromotive force, and consequently no tension.

The expression Electric Tension has been used by some writers in dif- ferent senses. In this treatise we shall always use it in the sense explained above,—the tension of so many pounds’, or grains’, weight on the square foot exerted by the air or other dielectric medium in the direction of the electro- motive force.