CHAPTER XIX.

VELOCITY OF ELECTRICITY—EXPERIMENTS—THE ELECTRIC EGG—FORCE OF THE ELECTRIC SPARK.

We are now acquainted with many facts concerning electricity, and have seen that electrical phenomena can be produced by the Electric Machine and the Leyden Jar. (An insulating stool—a stool with glass legs—is a very desirable adjunct for those who wish to experiment with the machine). Glass is a great insulator, or non-conductor, as a Russian philosopher found to his cost. He had an iron lightning-conductor from his house into his room, the end not connected with the earth but with a glass. One day the lightning came down the rod and reached the glass; had a communication been made with the earth by a chain, or directly, no mischief would have ensued. As it happened, however, the current was checked by the glass, and immediately darted towards him; it struck him in the head, and killed the poor man on the spot. If no insulating stool were used, the body charged would be discharged upon contact with the ground.

The velocity of electricity is very great, and experiments have frequently been made. Wheatstone undertook to ascertain the speed of the electric fluid, and the instrument he employed he called a “Chronoscope.” He caused a mirror to revolve with enormous velocity, and measured the speed by the vibrations of air, which produced a certain note by the same motive power. (We know already that certain notes are produced by a certain number of vibrations per second.) Wheatstone’s Chronoscope consisted of this mirror, in front of which was placed a circular block of wood, in which, in a row, were set six wires carrying small knobs; round these and over the wood he put an insulating varnish. A Leyden jar was connected outside with the first knob; between the second and third a quarter of a mile of copper wire was coiled, and a like length of wire between the fourth and fifth; the inside of the Leyden jar was then connected with the last knob, and the spark passed; it ran from knob to knob over the long coils of wire. If all the flashes over the wire and knobs occurred simultaneously the mirror would show them side by side; if not, as the mirror turned a trifle, the difference would be observed. The mirror did show a slight retardation in the passage of the flash, and from certain measurements and calculations Wheatstone estimated the velocity of the spark to be 288,000 miles a second. This rate will carry electricity round the earth in about a twelfth of a second, a rate Puck never dreamed of when he promised to “put a girdle round the earth in forty minutes.”

But it appeared from investigations subsequently made that it was not possible to express the velocity of electricity with any certainty, and a number of experiments were made as per the following table, with very different results. Sir William Thomson and Faraday endeavoured to account for these stupendous discrepancies, and the principles of retardation of electricity were established. The differences are shown below:—

Nature of
Wire.		Velocity
per Second.
Wheatstone’s Experiment in 1834Copper288,000
Gonella and FizeauCopper111,834
Iron62,130
MitchellIron28,331
WalkerIron18,639
GouldIron15,830
Astronomers in GreenwichCopper7,600
Asronomers in BrusselsCopper2,700
Result in Atlantic Cable, 1857Copper1,430
Result in Atlantic Cable, 1858Copper3,000

To account for the comparatively low velocities of the cables, Faraday proved that they act very much as a Leyden jar acts; that is, it takes time to fill, as it were, and to discharge them, the wire coating of the cable in air acting like the outer coating of the jar or the water in the case of an immersed cable, and the retardation observed is owing to resistance of conduction, and depends upon the way in which the electrical impulses traverse the wire. “There is a long, gradual swell, and still more gradual subsidence of the electric current, and the length of time that elapses between the initial impulse and the attainment of maximum strength, is proportional to the square of the length of the line.”

The duration of the electric spark has been calculated at the 1/24000 part of a second, but Professor Tyndall regards this as the longest or nearly the longest time it is perceptible; the shortest time is almost inconceivable. The brightest portion of a spark has been ascertained to last only forty-six millionths of a second, and certain experiments were made to ascertain the actual duration with various numbers of Leyden jars. It was discovered by Messrs. Lucas and Cuzin, by an application of the Vernier, with batteries consisting variously of two to eight jars, and obtained the following results[14]—

No. of Jars.Duration in
millionths of a Second.
226
441
645
847