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The laws of resistance are conveniently expressed by the following formula called Ohm’s Law.

C = E/R

where E=electromotive force in volts.

C=current in amperes.

R=resistance in ohms.

If two factors are known, the third can be found by substitution.

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The Electromagnet. The Solenoid.

If a current of electricity is passed through a copper wire, the wire will attract to itself iron filings, etc., as long as the current continues to flow. There is then a magnetic field around the wire. As soon as the current is shut off the filings drop away because the field immediately disappears with the cessation of the current.

FIG. 13. Magnetic Phantom about a Wire Carrying Current.

The lines of force flow around the wire in a circle. The circular lines of the field of force surrounding a straight wire may be shown by passing a wire vertically through a hole in the centre of a horizontal card. Iron filings are sifted over the card and a strong current passed through the wire. On tapping the card gently, the filings near the wire set themselves in concentric circles round it.