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.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.