383. =Electricity Discharged from Points.=--I have already, in giving an account of the electrical machine, spoken of the readiness with which electricity is received by points. It is discharged from them with equal readiness; so that, if a metallic point be attached to the prime conductor, the electricity will be carried off into the air nearly as fast as it is received upon the conductor. And as it passes off it creates a current in the air as it strikes upon it. The reaction of the air upon the electrical currents can be very prettily exhibited with the apparatus represented in Fig. 269, which consists of a cap, A, resting upon the point of a rod, and having pointed wires branching out from it in a wheel-like arrangement. You observe that the points are all bent one way. If this apparatus be set upright upon the prime conductor, the wheel can be made to revolve rapidly by working the machine. As the reaction of the air against the gases issuing from the rocket makes it rise, so the same reaction against the electricity issuing from these points causes the circular motion. If electricity be discharged from a point in a darkened room it appears like a brush of light, as represented in Fig. 270. [Illustration: Fig. 270.] [Illustration: Fig. 271.] 384. =Leyden Jar.=--The Leyden jar, Fig. 271, is so called because it was contrived at Leyden. It was suggested by an accidental result of an experiment tried there with the electrical machine. It consists of a glass jar coated upon the inside and the outside to near the top with tin-foil, and having a metallic rod passing through the cork, with one end touching the inner coating, and the other surmounted by a brass ball or knob. The jar is charged by holding the knob near to the prime conductor while the machine is worked. The electricity passes by the metallic rod to the inside coating of the jar, and accumulates there. This is positive electricity. In the mean time there is an accumulation of negative electricity on the outside coating. But how is this? It is by the repulsion of positive electricity for itself, and its attraction for its opposite, negative electricity. As you hold the jar in your hand positive electricity is repulsed from its outside through your arm earthward, while negative electricity is attracted to it by the positive which is within. The two fluids get as near to each other as possible. They are prevented from coming actually together by the non-conducting quality of the glass. If a slip of tin-foil were made to connect the inside foil with the outer, there would be no accumulation of electricity on the inside, for as fast as it passed from the prime conductor to the inside it would pass out over the bridge of foil to the outside, and down your arm and body to the earth. [Illustration: Fig. 272.]