Page:The New International Encyclopædia 1st ed. v. 10.djvu/684

INDUCTION COIL. The quantity of electricity moved at the break is equal to that moved at the make in the primary circuit; but, owing to the fact that the magnetism in the core vanishes much more rapidly at the break than it builds up at the make, the effect at the break is more compressed, and the potential, or electrical pressure, much greater than at the make. The suddenness of the break is of the greatest important, and most of the improvements in interrupters have been designed to make the break as sharp as possible. The important factors which affect the potential or length of spark produced by an induction coil are the relative number of turns of wire in the primary and in the secondary, the suddenness of the break, and the voltaic or potential of the current used in the primary coil.

Pohl in 1835 made a very crude induction apparatus of a different style; but the first to make large coils of the above typo were St and Rhuhmkorff, and from the latter is derived the name Ruhmkorff coil. which is frequently used as a synonym of induction coil. In 1855 Poggendoff increased the suddenness of the break by so arranging the device that the interruption occurred under an insulating liquid or in vacuo. (The cups B' ,A' in Fig. 2 have alcohol over the mercury.) In 1857 Ritchie, of Boston, devised a means of winding the secondary coil in sections, like circular disks, which, laid together and connected, formed the cylindrical coil. This obviated to a great extent the danger of a spark jumping across from one turn of the secondary to another through the insulation. Ruhmkorff was so impressed with the superiority of a Ritchie coil exhibited in Paris that he adopted the Ritchie method of winding, and it prevails at present. Poggendorff proposed, and in 1853 Jean tried with good success, the use of a liquid as the insulation of the secondary and at present most of the high-potential coils use liquid insulation. The advantage lies in the fact that if a spark should jump over, the hole in the insulation would immediately close. Fizcau suggested & decided improvement in 1853, introducing a con- denser at the break, as shown at F (Fig. 1 ). This enables the extra current of the primary to run into the condenser for an instant, while the points S and P are first separating, and by the time that F is charged P and S are too far apart for the current to jump across.

In 1869 a coil was built for the London Poly-technic Institute the core of which was 5 feet long, 4 inches in diameter, and weighed 123 pounds. The primary had 6000 turns of wire 0.095 inch in diameter and 3770 yards long, weighing 145 pounds. The wire of the secondary was 0.015 inch in diameter and 150 miles long. This coil gave a spark 29 inches long and could penetrate more than 3 inches of glass. The modern coil shown in Fig. 4 gives a spark 46 inches long.

The induction coil is especially used in the study of the electric discharge in rarefied gases, as in Geissler, Crookes, and X-ray tubes; also in electro-therapeutics, and more recently for X-rays and wireless telegraphy. Although the static machines have supplanted the induction coil in the best X-ray laboratories, it still holds its place in wireless telegraphy. It was by means of such a coil that Heinrich Hertz carried out his brilliant researches which led to the discovery of electric waves, thus confirming the great theoretical hypothesis of Maxwell, and laying the foundation for wireless telegraphy.

Consult: Alsopp, Induction coils and Coil-Making (New York, 189fi) ; Bonney, Induction Coils: A Practical Manual for Amateur Coil-Makers (New York, 1892) ; Induction Coils: How Made and Used, an American reprint of Dyer, Intensity Coils (New York, 1892) ; Hare, Construction of Large Induction Coils (London, 1900); Norrie, Induction Coil (New York, 1901 ) : Wright, The Induction Coil in Practical Work (New York. 1901). All of the large manuals of experimental physics also devote considerable space to 1his class of apparatus.

INDUCTION MOTOR. See

INDUCTOPHONE (from induct + Gk. phōnē, voice). A form of apparatus consisting of coils of wire and a telephone receiver, designed to afford communication between moving trains and stations. The apparatus and system were devised by Willoughby Smith, of London, in 1882, Init never have been practical- ly api>lied. Signals were transmitted inductively from coils of wire placed along the line to other coils in the train and received by means of a telephone.

INDULGENCE (Lat. indulgentia, indulgence, from indulgere, to indulge). A term of Roman Catholic theology, signifying the remission of temporal penalties due to sin, by one who has the power to distribute the spiritual treasures of the Church. An indulgence is not believed to remit guilt; only sacramental absolution avails for that; but it affects the punishment which would otherwise