Self Inductance

In 1832 Joseph Henry published his findings on the phenomenon of induced currents of a conductor on itself, later termed “self inductance,” and presented it to be published in the October edition of the American Journal of Science (AJS). In the article, he explored phenomena where sparks were produced when a circuit of wire driven by a galvanic cell was interrupted or broken.

“When a small battery is moderately excited by diluted acid, and its poles, terminated by cups of mercury, are connected by a copper wire not more than a foot in length, no spark is perceived when the connection is either formed or broken; but if a wire thirty or forty feet long be used instead of the short wire, though no spark will be perceptible when the connection is made, yet when it is broken by drawing one end of the wire from its cup of mercury, a vivid spark is produced. If the action of the battery be very intense, a spark will be given by a short wire; in this case it is only necessary to wait a few minutes until the action partially subsides, or no more sparks are given; if the long wire be now substituted, a spark will again be obtained. The effect appears somewhat increased by coiling the wire into a helix; it seems also to depend in some measure on the length and thickness of the wire. I can account for these phenomena only by supposing the long wire to become charged with electricity, which, by its reaction on itself, projects a spark when the connection is broken.” – (1)

The self inductance of a material is the response of a material to the changing magnetic field caused by either the interruption or resumption of electron flow within the conductor. The strength of this property is dependent upon various factors such as the resistance and arrangement of the conductor. He qualitatively determined the relative magnitudes of this newly discovered property in his later experiments by subjecting himself to the electric shocks as described:

“A handle of thick copper was soldered on each end of the large spiral at right angles to the ribbon, similar to those attached to the wires in Pixii’s magneto-electric machine for giving shocks. When one of these was grasped by each hand and the contact broken, a shock was received which was felt at the elbows, and this was repeated as often as the contact was broken.” – (2)

Using this method of measurement Joseph Henry discovered that uniformly coiled copper wire exhibits a stronger self inductance than straight wire of the same total length. Joseph Henry used numerous copper helices of varying lengths and thicknesses as described here.

Henry coil from Contribution number 3


Joseph Henry also discovered that using more turns of wire or in the copper helices used in the galvanic cell arrangement yielded more intense shocks and sparks. However there was a critical length or copper windings above which the intensity of the shocks began to decrease. Henry theorized that the increased resistance of longer wire began to counteract the self inductance of the wire (3).

(1) – Henry, Joseph. “On The Production Of Currents And Sparks Of Electricity From Magnetism.” American Journal Of Science & Arts 22.2 (1832): 403-408. American Antiquarian Society (AAS) Historical Periodicals Collection: Series 2. Web. 23 July 2012.
(2) – Henry, Joseph. “Contributions to Electricity and Magnetism. by Joseph Henry, Professor of Natural Philosophy in the College of New Jersey, Princeton, Late of the Albany Academy. no. II. on the Influence of a Spiral Conductor in Increasing the Intensity of Electricity from a Galvanic Arrangement of a Single Pair, &c.” Transactions of the American Philosophical Society 5 (1837): pp. 223-231. Print.
(3) – Henry, Joseph. “Contributions to Electricity and Magnetism. by Joseph Henry, Professor of Natural Philosophy in the College of New Jersey, Princeton. no. III. on Electro-Dynamic Induction.” Transactions of the American Philosophical Society 6 (1839): pp. 303-337. Print.