equator, a current would steadily flow through it. Experiments confirmatory of these views were made by Faraday himself;[1] but they do not strictly prove his hypothesis that the lines of force remain at rest; for it is easily seen[2] that, if they were to rotate, that part of the electromotive force which would be produced by their rotation would be derivable from a potential, and so would produce no effect in closed circuits such as Faraday used.
Three years after the commencement of Faraday's researches on induced currents he was led to an important extension of them by an observation which was communicated to him by another worker. William Jenkin had noticed that an electric shock may be obtained with no more powerful source of electricity than a single cell, provided the wire through which the current passes is long and coiled; the shock being felt when contact is broken.[3] As Jenkin did not choose to investigate the matter further, Faraday took it up, and showed[4] that the powerful momentary current, which was observed when the circuit was interrupted, was really an induced current governed by the same laws as all other induced currents, but with this peculiarity, that the induced and inducing currents now flowed in the same circuit. In fact, the current in its steady state establishes in the surrounding region a magnetic field, whose lines of force are linked with the circuit; and the removal of these lines of force when the circuit is broken originates an induced current, which greatly reinforces the primary current just before its final extinction. To this phenomenon the name of self-induction has been given.
The circumstances attending the discovery of self-induction
- ↑ Exp. Res., §§ 218, 3109, &c.
- ↑ Cf. W. Weber, Ann. d. Phys. lii (1841); S. Tolver Preston, Phil. Mag. xix (1885), p. 131. In 1891 S. T. Preston, Phil. Mag. xxxi, p. 100, designed a crucial experiment to test the question; but it was not tried for want of a sufficiently delicate electrometer.
- ↑ A similar observation had been made by Henry, and published in the Amer. Jour. Sci. xxii (1832), p. 408. The spark at the rupture of a spirally-wound circuit had been often observed, e.g., by Pouillet and Nobili,
- ↑ Exp. Res., § 1048.
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