Michael Faraday, His Life and Work

by Silvanus P. Thompson

The ninth series of electrical researches occupied the autumn of 1834. In it he returns to the study of the magnetic and inductive actions of the current, investigating the self-induced spark at the break of the circuit, to which his attention had been directed by Mr. W. Jenkin. Several points in this research are little known even now to electricians, the laboratory notes being much more detailed than the published paper. He describes an exceedingly neat high-speed break for producing rapid interruptions, using for that purpose stationary ripples on the surface of a pool of mercury. In a wonderful day’s work on 13th November, filling thirty-four pages of the laboratory book, illustrated with numerous unpublished sketches, he tracks out the properties of self-induction. He proves that the spark (on breaking circuit) from a wire coiled up in a helix is far brighter than that from an identical wire laid out straight. He finds that a non-inductive and, therefore, sparkless coil can be made by winding the wire in two opposite helices. “Thus the whole [inductive] effect of the length of wire was neutralised by the reciprocal and contrary action of the two halves which constituted the helices in contrary directions.” The next day he writes: “These effects show that every part of an electric circuit is acting by induction on the neighbouring parts of the same current, even in the same wire and the same part of the wire.”

EFFECTS OF SELF-INDUCTION.

On 22nd November he is trying another set of experiments, also never fully published. They relate to the diminution of self-induction of a straight conductor by dividing it into several parallel strands at a small distance apart from one another. The note in the laboratory book runs thus:—

Copper wire 1/23 of inch in diameter. Six lengths of five feet each, soldered at ends to piece of copper plate so as form terminations, and these amalgamated. When this bundle was used to connect the electro-motor it gave but very feeble spark on breaking contact, but the spark was sensibly better when the wires are held together so as to act laterally than when they were opened out from each other, thus showing lateral action.

Made a larger bundle of the same fine copper wire. There were 20 lengths of 18 feet 2 inches each and the thick terminal pieces of copper wire 6 inches long and ⅓ of inch thick.

Fig. 13.

This bundle he compared with a length of 19 feet 6 inches of a single copper wire ⅕ inch in diameter, having about equal sectional area. The latter gave decidedly the largest sparks on breaking circuit.

Faraday did not see fit at this time to accept the idea, suggested indeed by himself in 1831, that these effects of self-induction were the analogue of momentum or inertia. That explanation he set aside on finding that the same wire when coiled had greater self-inductive action than when straight. Had he at that time grasped this analogy, he would have seen that the very property which gives rise to the spark at break of circuit also retards the rapid growth of a current; and then the experiment described above would have shown him that Sir W. Snow Harris was right in preferring flat copper ribbon to a round wire of equivalent section as a material for lightning conductors. He was, however, disappointed to find so small a difference between round wires and parallel strands. The memoir as published contains an exceedingly interesting conclusion:—

Notwithstanding that the effects appear only at the making and breaking of contact (the current, remaining unaffected, seemingly, in the interval,) I cannot resist the impression that there is some connected and correspondent effect produced by this lateral action of the elements of the electric stream during the time of its continuance. An action of this kind, in fact, is evident in the magnetic relations of the parts of the current. But admitting (as we may do for the moment) the magnetic forces to constitute the power which produces such striking and different results at the commencement and termination of a current, still there appears to be a link in the chain of effects—a wheel in the physical mechanism of the action, as yet unrecognised.