The Leyden Jar requires a little more detailed description, as it is to it we are indebted for our Battery. It is a common glass bottle or jar, coated both inside and out with tinfoil nearly as high as the shoulder, _a_ _a_. The mouth should be firmly closed with a bung of wood, _g_ _g_; a hole should be bored in the bung, through which a brass rod is tightly pushed. The rod, too, is topped by a brass knob, and a brass chain is attached to the other extremity. The interior of the tinfoil receives positive electricity, and the exterior negative when the jar is charged from the “Electrophorus.” To discharge the jar and create a _shock_ it is necessary to put one hand on the outside, and the other on the knob of the jar. A brilliant spark and a severe shock will result if the jar has been fully charged. It is as well to be cautious when trying this experiment. The effect of the shock may be felt by any number of persons joining hands, if one at one end of the row, and one at the other end, touch the knob and the outside of the jar simultaneously.
[Illustration: Fig. 210.—The Leyden Jar.]
This electric discharge is lightning in miniature, and it is to Benjamin Franklin that the world is indebted for the discovery. The philosopher was greatly interested in the science of Electricity and, having retired from business, he devoted himself to the consideration of thunderstorms. He wrote a treatise to show that points drew off electricity, and that electricity and lightning were similar. He urged that metallic rods might be attached to ships and buildings, so that during thunderstorms, or at other times, the electricity might be harmlessly carried into the ground. This suggestion he made without being able to explain _why_ points did carry off electricity without a spark. The reason is because there is no place to store it; it runs away at once, without having time to collect, as in a ball.
Franklin made one or two experiments before his renowned kite-flying arrangement, which convinced him that electricity was by no means an agent to be played with. He endeavoured to kill a turkey by electricity, but by incautious handling of the jars in which the “fluid” was stored, he discharged them, and describes the result: “The flash was very great, and the crack was as loud as a pistol; yet my senses being instantly gone, I neither saw the one nor heard the other, nor did I feel the stroke on my hand, though I afterwards found it raised a round swelling where the fire entered as big as half a pistol bullet.” On a subsequent occasion he was again struck senseless while endeavouring to administer a shock to a paralytic patient.