The quantity of electricity moved at the break is equal to that moved at the make in the pri mary circuit; but, owing to the fact that the magnetism in the core vanishes much more rap idly at the break than it ln'ilds up at the make, the effect at the break is more compressed, and the potential. or electrical pressure, inueh greater than at the make. smith- of is of the greatest importance. 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 rebuke number of turns of wire in the primary and in the secondary, the suddenness of the break, and the voltage or potential of the current used in the primary coil.
Pohl in 1835 made a very crude induction ap paratus of a different style; but the tirst to make large coils of the above type Were Stiihrer and Rutunkorlf, and from the latter is derived the name `Itulunkortf coil,' which is frequently used as a synonym of induction coil. In 1855 Pog geneholf increased the suddenness of the break by so arranging the device that the interruption occurred under an insulating liquid or in vacuo.
(Tie cups A' in Fig. 2 have alcolnd 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 con nected, 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. Ruhmkorlf was so im pressed with the superiority of a Ritchie coil xhibited 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 in sulation of the secondary. and at present most of the high-potenti•l 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. Fizeau suggested a.
decided improvement in 1853, introducing a con denser at the break, as shown at (Fig. 1). This enables the extra current of the primary to run into the condenser for an instant, Willie tile points S 111111 1' .1 re first separating, and by the time that I. is charged 1' :and S are too far apart for the current to jump across.
In 1869 a coil was built for the London Poly techni• Institute the core of wlriclr was 5 feet long, -t inches in diameter, and weighed 123 pounds. The primary had 6000 turns of \vire 0.095 inch in diameter and 3770 yards weighing 145 pounds. The wire of the was 0.015 inch in diameter and 150 miles long. This cull gave a spark 29 inches long and could penetrate more than 3 inches of glass. The mod ern 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, ('rookes, and X-ray tubes; also in clectro-therapeuties, and more recently for X-rays and wireless telegraphy. Although the static machines 'nave 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 Ilertz 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 roil Making (New York, 1896) ; Bonney, induction Coifs: it rractiral Manual for Amateur Coil Makers (Ne• York, 1892) ; induction. Coils: More Made and Used, an American reprint of Dyer, Intensity Coils (New York, 1892) ; Hare, Const ?fterion of Large Induct ion Coils (London, 1900) ; Norris, Induct ion Coil (New York, 1.901); Wright, The Induct ion ('oil in Pro et iral Work (New York, 1901). All of the large manuals of experimental physics also devote considerahle space to this class of apparatus.