31. A B is the glass tube, clasped by the rubber, R. P P' are strips of metal furnished with rows of points. From P P' wires proceed to the knob C, which is insulated by the horizontal stem, G, from the stand of the machine. This insulating stem may be abolished with advantage, the wires from P and P' being rendered strong enough to support the ball C. At C sparks may be taken, a Leyden-jar charged, the electric mill turned, while wires carried from it may be employed in experiments on ignition.
Fig. 31.
"Seldom," says Riess, "has an experiment done so much to develop the science to which it belongs as this of the ignition of bodies by the electric spark." It aroused universal interest: the experiment was repeated in all royal houses. Money was ready for the further prosecution of electrical research. The experiment afterward spread among the people. Klingenstierna astonished King Frederick of Sweden by igniting a spoon of alcohol with a piece of ice. Riess considers it probable that the general interest thus excited led to the discovery of the Leyden-jar, which was made soon afterward.
Cadogan Morgan, in 1785, sought to produce the electric spark in the interior of solid bodies. He inserted two wires into wood, and caused the spark to pass between them: the wood was illuminated with blood-red light, or with yellow light, according as the depth at which the spark was produced was greater or less. The spark of the Leyden-jar produced within an ivory ball, an orange, an apple, or under the thumb, illuminates these bodies throughout. A lemon is especially suited to this experiment, flashing forth at every spark as a spheroid of brilliant golden light. The manner in which the lemon is mounted is shown in Fig. 32. The spark occurs at s. A row of eggs is also brilliantly illuminated throughout at the passage of every spark from a Leyden-jar.
