. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . unaltered (Fig. 38). While it is not possible to observe thelines in the material of the iron itself, they may be supposed to passthrough the iron without break of continuity, as in the figure. Todistinguish these lines from the lines of force without the iron they arecalled lines of induction. They may be regarded as the addition of thelines of force of the molecular magnets to those of the inducing total number of lines passing through a given area whether of airor of iron is called the
. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . unaltered (Fig. 38). While it is not possible to observe thelines in the material of the iron itself, they may be supposed to passthrough the iron without break of continuity, as in the figure. Todistinguish these lines from the lines of force without the iron they arecalled lines of induction. They may be regarded as the addition of thelines of force of the molecular magnets to those of the inducing total number of lines passing through a given area whether of airor of iron is called the magnetic flux through the 66 ELECTROMAGNETISM The effect upon the field of the iron is to concentrate the lines initself as if they pass more readily through iron than through this reason, this property of iron is called permeability. It is meas-ured by the ratio of the magnetic flux through a bar of iron placed ina field to the flux through the same space when the iron is value differs with different specimens of iron, and is often as high as1000 or more. Fig. 38. Lines of induction. The total effect of increasing the flux due to the permeability of ironin a field is not obtained unless the whole path of the lines of inductionis through the iron. Even a small air gap materially reduces theamount. For this reason the most powerful magnets are constructedof horseshoe form in order to bring the poles closer, and the circuitis completed by a piece of soft iron called an armature or keeper. MAGNETIC ACTION OF CURRENTS The Magnetic Field of a Current.—It was discovered by Oersted in1820 that a current of electricity affects a magnet in its neighborhood;in other words, that a current produces a magnetic field. The effect isdue to the flow of the electricity, as no static charges, however great,produce any such effect. The force exerted upon a magnetic pole isnot one of attraction or repulsion, but is at right angles to the linejoining it to the conductor
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