ELECTIUC In the neighborhood of any charged body, forces are noticed if other charges are brought near or moved about it is said to lie surrounded by a 'field of force.' The 'direction' of the field at any point is defined as that in which a particle of matter positively charged would move, if placed at that point and left free to move. If a line is drawn such that at each of its points its direction gives that of the field, it is called a 'line of electric force.' Suet a line evidently starts from a positive charge and ends on a negative one: there is therefore no line inside a closed conductor owing to its charge: further, these lines are perpen dicular to the surface of a charged conductor: for, if they were not• there would he a component of the force in the surface itself and consequently there would be motion of the charges. because conductor is such a body that there is no opposi lion to a force tending to move a charge: and this would be contrary to the assumption that the charges are at rest.
The work required to bring a particle with a unit positive charge from the surface of the earth to any point in the field of force round a charged body is called the 'electric potential' at that point. (The earth is taken as the starting point heeause it is a huge conductor whose elec tric condition may be assumed to be steady: and this definition of potential is equivalent to defining the potential of the earth its being O. Potentials are measured with reference to that of the earth, just as temperatures on the centi grade scale arc measured with reference to that of inciting ice.) If one point has a higher po tential than another. it requires work to tarry a unit positive charge from the latter point to the former: and hence a force must have been over come: in other words. lines of force always pass from points of high to point: of lower potential. It is evident, further. that the potentials of all of a charged conductor and of all points inside are the :nine. Therefore the motion of a positive charge is always from high to low po tentials, and that of a negative charge is in the opposite direction.
If a positive charge is separated from a nega tive one by some medium such as air. glass. or
paper, and if the charges are large, the difference of potential between any two points of the field which are (dose together will also he large, and there will he a strong force tending to more posi tive and negative charges in opposite directions.
This force may become so great that there is a rupture of the material m(11111111, air, glass, or palier, and a 'spark' is seen. The principal action in the spark is to make the medium conducting; and the positively and negatively charged bodies are joined by a conductor and totally or in part discharged. There are other effects of the spark. notably the thermal and luminous ones. Two conduet ors of similar shape, separated from each other by a thin layer of dielectric, form what is called an 'electric • tenser': be cause, if one is charged positively and the other with an equal quantity of negative electricity, the potential of the former is lower than it would be if the hitter conkluctor were absent, and there fore it can be eharged with a greater quantity, with less danger of sparks passing off to the earth and thus discharging it.
Common forms of etindensers (see Cox DENS ERS ) are parallel plates of tin-foil separated by glass or paraffined paper, coaxial cylinders, etc. One of the most familiar types is the 'Lydell jar,' which consists of a glass bottle eoated inside and out, except near the opening, with tin-foil.
It is observed that if a conductor is charged which has sharp points the charge rapidly dis appears. This is owing to the fact that the fall of potential from the conductor into the air is most rapid at the points, and thus minute sparks pass off to the air, giving charges to the particles in the air which are then repelled. Similarly, if a charged body is brought near an uncharged in sulated conductor which has sharp points turned toward the charged body, the latter induces charges on the conductor. Those on the side near the charged body pass off the points, are carried across, and discharge the former, leaving a charge on the conductor of the same kind as that on the body which was originally charged.