Identifier: archivesofphysic04ameruoft (find matches)
Title: Archives of physical medicine and rehabilitation
Year: 1920 (1920s)
Authors: American Academy of Physical Medicine and Rehabilitation American College of Radiology and Physiotherapy American Congress of Physical Medicine and Rehabilitation Radiological Society of North America
Subjects: Physical therapy
Publisher: (Chicago, etc.)
Contributing Library: Gerstein - University of Toronto
Digitizing Sponsor: University of Toronto

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he Principle of Electro-Magnetism Most readers are familiar with thefact that when a small bar magnet isplaced beneath a sheet of paper, andiron filing are sprinkled on it, the filingsarrange themselves along regular linessomewhat as represented in Figure 2.This simple experiment indicates thatin the whole region around the magnetthere is what is called a magnetic fieldof force. To visualize this field wegenerally say that it is traversed by magnetic lines of forces, the closenessof the lines of force at any particularplace being a measure of the strengthof the magnetic field at that place.These lines of force are closed curvesleaving the N pole of a magnet, enter-ing the S pole and we say there is amagnetic flux through the magnet. In-deed, whenever magnetic lines arepassing through any region, we speakof a magnetic flux through that region.Suppose, now, that the wire AB(Fig. 3) is carrying a current of sev-eral amperes, and we sprinkle iron fil-ings on a sheet of paper through which
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>2 X-RAYS AND X-RAY APPARATUS—ROBERTSON the wire passes. We find that, so longas the current is flowing, the filings arearranged in circular lines with the wireas centre. A magnetic field, therefore,surrounds a Tvire carr\)ing a current.This is the fundamental principle ofelectro-magnetism. We can have mag-netic fields subject to the control of anelectric current. If a wire carrying acurrent is wound into what is called asolenoid (Fig. 4) it can easily beshown that one end of the solenoid actsas the north, the other end as the southpole of a bar magnet with magneticlines of force somewhat as representedin the figure. Obviously these lines arelinked with the electric circuit, and atonce disappear when the electric cir-cuit is broken. If the air inside the coilbe replaced by soft iron, the soft ironbecomes strongly magnetized under theinfluence of the magnetic field due tothe current, and the number of linesof force may be increased many times.We have, in fact, an electro-magnet.Br

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