. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . coil. The current thus passes around the coil first in one direction and then in the other, and nomagnetic effect is produced. 68 ELECTROMAGNETISM The Electromagnet.—If a core of soft iron is placed inside a solenoidthrough which a current is flowing, it becomes magnetized by inductionfrom the magnetic field of the solenoid. On account of the greater per-meability of the iron, the magnetic flux through the solenoid is consid-erably increased. In this way a powerful magnet is obtained, which,however
. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . coil. The current thus passes around the coil first in one direction and then in the other, and nomagnetic effect is produced. 68 ELECTROMAGNETISM The Electromagnet.—If a core of soft iron is placed inside a solenoidthrough which a current is flowing, it becomes magnetized by inductionfrom the magnetic field of the solenoid. On account of the greater per-meability of the iron, the magnetic flux through the solenoid is consid-erably increased. In this way a powerful magnet is obtained, which,however, loses its strength almost completely when the exciting currentis interrupted. Such a device is called an electromagnet, and is muchstronger than a permanent magnet of the same size. Electromagnetsare extensively used not only where powerful magnetic fields are desired,as in the field magnets of dynamos, motors, etc., but also where amechanism is to be controlled at a distance, as in the telegraph, electricbell, etc., the simple closing of a switch bringing the magnet into action. Fig. 41. Lines of force of a solenoid. Action of a Magnetic Field on a Current.—Corresponding to the forceexerted by a current upon a magnet, there is a reciprocal action of a mag-netic field upon a current. This may be observed if the current insteadof the magnet is free to move. Thus, if a coil carrying a current is hungin a magnetic field so as to be free to move, it will turn so that its planewill be at right angles to the lines of force of the field. Again, a solenoidif free will place its axis in the direction of the field. The general lawis that the coil moves so as to include as many lines of magneticforce as possible. In applying this law, the positive direction of theforce is related to that of the current by the screw rule. Since a stream of charged particles is equivalent to a current, thisalso will suffer a deviation when in a magnetic field. It is by this meansthat it has been shown
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