ELECTRICITY. 755 ELECTRICITY. sions of a charge are ( KMLT"') ^ or Jl' L' T K'- L'nUl soinellmig is known of the nature of K, there tan be no mechanical understanding of an electrical charge.) Tluit portion of this law of electrostatic action which says that the force varies as ec' and inversely as r", is known as "Coulomb's law.' It lius been tested experiment- ally in two ways: one directly, by placing charges at different distances : the other indi- rectly, for it can be proved mathematically that only if this law is true will there be no elec- trical force inside a closed hollow conductor, e.g. a spherical shell. Coulomb (17S5) applied the first method, which is not accurate : and the second was used by Cavendish (1773) and later by Clerk ilaxwell. The fact that the force de- pends upon the medium was discovered by Caven- dish (about 1772), and later independently by Faraday, who thoroughly investigated the sub- ject. Electric Potenti.m.. In the neighborhood of any charged body, forces are noticed if other charges are brought near or moved about; it is said to be 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.' Such 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 be a component of the force in the surface itself and consequently there would be motion of the charges, because a conductor is such a body that there is no opposi- tion 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 because 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 as being 0. Potentials are measured with reference to that of the earth, just as temperatures on the centi- grade scale are measured with reference to that of melting ice.) If one point has a higher po- tential than another, it requires work to carry 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 points of lower potential. It is evident, further, that the potentials of all points of a charged conductor and of all points inside are the same. 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 close together will also be large, and there will he a strong force tending to move posi- tive and negative charges in opposite directions. This force may become so great that there is a rujiture of the material medium, air, glass, or paper, and a "spark' is seen. The principal action in the spark is to make the medium conducting; and so 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 conductors of similar shape, separated from each other by a thin layer of dielectric, form what is called an "electric condenser' : be- cause, if one is charged positively and the other with an equal quantity of negative electricity, the i)otential of the former is lower than it would be if the latter conductor were absent, and there- fore it can be charged with a greater quantity, with less danger of sparks passing oil' to the earth and thus discharging it. Common forms of condensers (see CoKDEXS- ERS) are parallel plates of tinfoil sepaiated by glass or paraffined paper, coaxial cylinders, etc. One of the most familiar types is the "Lydcn jar.' which consists of a glass bottle coated 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 oft' to the air, giving charges to the particles in the air which are then repelled. Similarly, if a charged body is brought near an unchaiged 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. Cap.city. If the charge on an isolated con- ductor is increased, so is its potential, because more work would be required to bring up a unit charge: if the charge is doubled, so is the poten- tial. In mathematical language, the charge equals a constant times the potential. If e is the charge and V the potential, e ^ CV. This constant C depends naturally on the shape and size of the conductor and on the surroundiiig dielctric; it is called the 'capacity' of the con- ductor. Similarly, if a condenser is charged with +e and — e, and if the potentials of the two conductors are V, and V„. there will be a constant relation between e and V, — V;, which may be expressed e=C(V, — V,) . This con- stant C depends upon the shape, size, and distance apart of the two conductors and on the nature of the dielectric which separates them: it is called the 'capacity of the condenser.' It was first observed by Cavendish and later by Faraday that the capacity of a condenser was different for different dielectrics, e. g. air. glass, etc. Thus, if the same charge is given two con- densers which are identical except in that one has air for its dielectric and the other glass, it is observed that the difference of potential is greater in the former than in the latter, thus showing that the capacity of the latter is the greater. The ratio of the two capacities was called by Faraday the 'specific inductive ca- jiacity' of the glass with reference to air. If. in- stead of using air as the dielectric, there had been a vacuum, the ratio of the capacity of the glass condenser to the vacuum one would be a
