Ruhmkorff coil(induction coil)ملف رومكورف - ملف الحث

The induction coil is an application of electromagnetic induction from which a p.d. of several thousand volts in one direction can be obtained . it was invented by Ruhmkorff about 1836 .
http://www.physics.gla.ac.uk/~kskeldon/PubSci/exhibits/E4/pinduct1.gif
the figure above illustrates the essential features of the apparatus . a battery is connected with series with primary coil of possibly 50 turns with a key is connected with series with the circuit . the coil is made of thick copper wire and contains a bundle of soft iron wires . the circuit has made –and-break arrangement , as in the electric bell , provided by a soft iron hammer-head supported by a steel spring and platinum contact . a capacitor is connect with parallel with the key .
when the circuit is made by the key a current is obtained in the primary coil . the iron hammer-head is then attracted towards the primary coil . when the circuit is immediately broken and the iron hammer-head falls back to renew contact . this intermittent to and fro movement of the iron hammer-head goes on at a definite frequency , and the current in the primary coil therefore rises and falls rapidly . the number of lines of force inside the coil thus keeps changing rapidly , and a condition is therefore obtained in which an induced e.m.f. may be produced . for this purpose a secondary coil is wound over the primary coil . the secondary coil has much larger number of turns than the primary coil , perhaps several thousand , and is therefore made of thinner wire than the primary coil , from which it is insulated .
the p.d. at the secondary terminals X,Y is first one direction and then in the opposite direction , since the change in the secondary lines occurs in opposite directions as the primary coil made and broken . experiment shows , however , that the p.d. at the break is several thousand volts while the p.d. at the make is a few hundred volts , which is explained by the much shorter time the change takes in the former case . on the average , then , the p.d. at X , Y is high and in one direction . the induction coil is used to provide a source of high e.m.f. for car radios , and is employed in the laboratory to operate gas-discharge tubes , which are similar to the neon tubes used for sign lighting.