If a piece of copper wire through which a current of electricity is flowing is passed through a cardboard or glass plate and the card or plate is sprinkled with iron filings, the filings arrange themselves in circular lines (Fig. 71). If the card or plate is jarred and the iron filings displaced, they will rearrange themselves in the same circular lines.
FIG. 71. - Lines of Magnetic Force Around an Electric Wire.
If, on the other hand, the current is not flowing, the filings will not assume circular lines. This shows that there is a definite relation between magnetism and electricity. When a conductor passes through a magnetic field in the proper direction, it produces a current of electricity, or when a current passes through a conductor it produces a magnetic field.