a b. Leyden jar coated with tinfoil, and standing on any non-conductor, such as gutta percha or the resinous or glass plate, c.
Sixth Experiment.
When the Leyden jar is coated with spangles of tinfoil, a spark appears at each break, and the whole jar is lit up with hundreds of brilliant sparks each time it is charged and discharged, and as this occurs with amazing rapidity, the light is almost continuous. (No. 1. Fig. 228.) The larger the Leyden jar, the shorter the spark, and vice versâ. By the employment of a nicely-made screw and inch-scale, the distance between the discharging points connected with a Leyden jar can be accurately determined; and Mr. Hearder states that supposing a Leyden jar has one square foot of charging surface, it will give a spark of one inch in length, but if a smaller jar is used, with only half a square foot of charging surface, the spark would be about one inch and a quarter in length. (Fig. 228.)
Fig. 228.
No. 1. Spangled Leyden jar. No. 2. Hoarder's apparatus for measuring the length of spark for Leyden jar and coil. p p. Glass pillars. No. 3. Two best forms of spangles to paste on a Leyden jar.
Seventh Experiment.
The direction and rapidity of the current appear to influence greatly the heating and fire-giving power of the coil, and the following experiment, devised by Mr. Hearder, furnishes a curious illustration of this fact.
When the current passes in the direction of the arrows (Fig. 229), the platinum wire remains perfectly cool whilst the gunpowder is fired; and the contrary takes place if the current is reversed—viz., the gunpowder does not blow up, but the platinum wire is heated. In the second experiment, a Leyden jar is included in the circuit. (Fig. 229.)