The Induction Coil
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-400
2025-12-13
23
The Induction Coil or Ruhmkorff coil, is a combination of coils used for the purpose of getting induced currents of high potential difference. Figure 1 is a diagram showing the relation of the parts in an induction coil. The essential parts are a soft iron core, a primary coil of large insulated wire connected with the battery, a secondary coil of a very much larger number of turns of, fine insulated wire connected with binding posts A and B, an automatic make-and break arrangement at P, between the primary coil and the battery, a condenser C connected with the primary circuit on each side of P, and a switch S.
The operation of the coil is as follows: When the switch is turned on, the current passes through the primary and makes a magnet of the iron core. This attracts the soft iron armature, which is fastened to a light spring, and breaks the current at P. As soon as this is done, the core is no longer a magnet, the armature is thrown back by the spring, the contact is again made at P, and the action is repeated. When the current is made in the primary coil, a current in the opposite direction is induced in the secondary, and when the current in the primary is broken there is an induced current in the same direction in the secondary. The self-induction of the primary when the current is made acts against the battery current flowing in it, and reduces the induced E.M.F. in the secondary; but when the current is broken, the self-induction acts with the battery current and increases the E. M. F. of the secondary. The effect of the condenser is to increase the саpacity of the primary coil and to shorten the time of breaking, thus raising the E. M. F. of the secondary. It also discharges through the battery, immediately after the current is broken, in a direction contrary to the battery current; this helps to demagnetize the core quickly. Since the current in the primary drops from a maximum to zero much faster than it rises from zero to maximum, the induced E. M. F. in the secondary is correspondingly higher at the break than at the make.
The induced current is of high voltage because the total number of cuttings of the lines of force is made very great - in two ways: the lines of force of the primary coil are increased in number by the presence of the iron core; and the secondary coil is made up of a very large number of turns of fine wire, thus increasing the number of times that each line of force is cut.
Since the E. M. F. rises so high in an induction coil the insulation should be as nearly perfect as possible. In modern coils the secondary is wound in sections, and these thoroughly coated with insulating wax under conditions that secure the removal of all air.
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