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 Velocity per Wire. Second. Wheatstone’s Experiment in 1834 Copper 288,000 Gonella and Fizeau { Copper 111,834 { Iron 62,130 Mitchell Iron 28,331 Walker Iron 18,639 Gould Iron 15,830 Astronomers in Greenwich Copper 7,600 “ Brussels Copper 2,700 Result in Atlantic Cable, 1857 Copper 1,430 “ “ 1858 Copper 3,000
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]—
Duration in No. of Jars. millionths of a Second. 2 26 4 41 6 45 8 47
“So,” adds the writer, “the duration (of spark) increases in proportion with the number of the jars. It increases also with the striking distance, but is independent of the diameter of the balls or globes between which the spark strikes.”