INDUCTION COIL, one of the early electrical instruments, that simply illustrates the principle of induction, consisting of an iron core about which two wires are differ ently coiled: now used mainly for study, and in connection with high frequency currents, radiography and X-ray investigations. (See 'minx-now). The essential features of the in strument are represented, diagrammatically, in the accompanying cut. A soft iron core is surrounded by two coils 'of wire, the finer one outside of the other. The outer coil is the one in which the high tension induced currents arc produced; it is called the "secondary coil," and is not in direct electrical connection with any other part of the instrument. The inner coil, which is called the primary coil, is of coarse wire, wound close to the iron core, and is con nected to a battery, so that when the current from the battery is flowing the primary coil causes the iron core to become magnetised. As is explained in the article INDUCTION, no current is generated in the secondary coil, so long as the magnetism of the iron core re mains constant; but whenever the magnetism of the core increases or diminishes, a current is produced in the secondary. A device called a "break') or ter ru pt e is therefore pro vided, so that the magnetism of the iron core may be rapidly established and destroyed. The commonest form of interrupter is that indicated in the cut, which does not call for special ex planation, since it is used in electric hells and other simple forms of electrical apparatus. The Wehnelt electrolytic interrupter is greatly in favor among physicists, however, and is now often used in connection with induction coils, especially when they are to be run with the commercial current used for incandescent lighting, where the potential difference that is used is something over 100 volts. In the Weh nelt interrupter the primary coil on the iron core is made of a few turns of relatively thick copper wire, its purpose being solely to effect the magnetisation of the core; but the sec ondary coil, in which the induced currents are generated, is made of fine wire, and in order to multiply the inductive effect as far as possi ble, the secondary is made of great length, often containing many miles of wire. In the
celebrated Spottiswoode coil the secondary con tained no less than 280 miles of wire. The primary coil, being wound directly upon a soft iron core, commonly has a very considerable amount of self-induction, so that when the circuit is broken by the interrupter the current does not suddenly cease, but continues to flow across the interval at the break for an ap preciable fraction of a second, as is readily seen by the strong sparking that occurs at the moment of interruption. In order to reduce the sparking as far as possible, a condenser of suitable capacity is provided, so that when the break is made in the circuit, the cafter-currents due to the self-induction of the primary can discharge into the condenser, instead of passing across the break in the circuit and causing a spark. The condenser causes the interruption of the current in the primary coil to be much more sudden, and it materially increases the potential that is developed in the secondary coil, since this is proportional to the rate of variation of the magnetism of the core, and is much greater when the magnetism falls off abruptly than when it persists for an appre ciable fraction of a second after the break has been made.
The induction coil was brought into some thing like its present form by Ruhmkorff, and is frequently known, in consequence, by his name. Improvements in the winding of the secondary coil were introduced by Ritchie of Boston about 1857. Ritchie's most important improvement consisted in disposing the sec ondary wire in sections, which were so related to one another that the risk of internal dis ruptive discharge through the coil itself might be reduced to a minimum. Induction coils are very generally used in studying the discharge of electricity through gases, for exciting X-ray tubes and in connection with the high potentials required in wireless telegraphy. Consult Bon ney, 'Induction Coils' ; Alsopp, Coils and Coil-Making' Wright, The Induc tion Coil in Practical Work.>