Magnets are of two shapes: straight or bar (Fig. 68) and horseshoe (Fig. 69). In every magnet there is a limited space surrounding each end or pole in which its magnetic properties are exhibited. This is called the magnetic field. If, for example, magnetized iron filings are sprinkled over a sheet of paper, they will assume curved lines, bringing into view a few of what are called the lines of force of a magnetic field. The portion of this magnetic field that is the strongest is assumed to contain the greatest number of lines of force

Fig. 68.   Bar Magnet with Iron Filings.

Fig. 68. - Bar Magnet with Iron Filings.

Fig.69.   Horseshoe Magnet with Iron Filings.

Fig.69. - Horseshoe Magnet with Iron Filings.

The total number of lines of force which pass through a field is called the magnetic flux. The magnetic flux always flows in a complete circle or circuit. The material through which it flows affects variously the resistance offered to the free passage of the flux.