Fig. 216.—Experiment with card.
In the same manner a glass may be perforated if the current be stronger. Of course the whole apparatus—and particularly the plate—must be quite dry, and it will be better to put a drop of oil under the upper needle point so as to prevent the electricity spreading over the glass. The glass will be found pierced by the electric spark when the Leyden jar is brought into requisition (fig. 217).
It was at one time contended that the sudden expansion of the air by the electric spark caused heat to be generated, and a thermometer was invented by Kinnersley to show this. The illustration (fig. 218) will explain it. When the electric spark passed between the balls in the large tube the water rose in the smaller. But the immediate return of the water to its level showed that the disturbance was only mechanical, and not owing to heat.
Fig. 217.—Experiment with glass.
Fig. 218.—Kinnersley’s thermometer.
We may now pass from the “frictional” to the other kind—viz., “dynamical” electricity, and we shall begin with the consideration of Galvani’s experiments and the Voltaic Pile.
