Magnetic flux pattern due to a current in a long straight wire
Since an electric current has a magnetic effect, we should expect it to be surrounded by magnetic flux. The following experiments show that this is so. A large rectangular coil consisting of about twenty turns of insulated copper wire is set up vertically with one of its vertical sides passing through a hole in the centre of a piece of cardboard supported horizontally (Fig, 36.3). A current of about 3 A
may be passed through the coil from a 6 V battery in series with a rheostat, an
ammeter and a switch. The reason for having a coil of twenty turns instead of one is that a single wire would require a current of 60 A to produce the same effect. This would need not only a very large battery but the wire would also have to be very thick indeed, or it would become exceedingly hot when the current was passing.
A fine layer of iron filings is then sprinkled on the cardboard; the current is switched on and the card tapped gently. The filings set in a series of concentric circles about the wire as centre. A small plotting compass placed on the card indicates the direction of the flux. If the current is reversed by changing over the battery connections the compass needle will swing round and point in the opposite direction, but the pattern of the flux remains unaltered. James Clerk Maxwell, a great mathematical physicist of the nineteenth century, gave a rule relating the direction of the magnetic flux round a wire to the direction of the current flowing through it. This is known as the “screw rule”, and is stated as