The Day the Universe Changed - James Burke [138]
Faraday’s induction machine. The small cylinder attached to a battery acts as an electromagnet; when it is moved in and out of the larger cylinder, which is attached to a galvanometer, the wire round the larger cylinder becomes electrified.
Using a galvanometer, an instrument consisting of a sprung compass needle which Oersted had used to take readings of the amount of magnetic deflection caused by a current, Georg Ohm was able to quantify the conductivity of metals and the flow of current in terms of resistance in the wire. Something was now known about the way the current moved and it seemed to show that the currents and the magnets interacted. A current made a wire magnetic, so could a magnet make electricity? In 1821 in England a laboratory assistant at the Royal Institution called Michael Faraday examined the possibility, saying: ‘I have long held an opinion… that the various forms under which the forces of matter are made, manifest one common origin… so directly related and mutually dependent that they’re convertible, as it were, one into another… ’
Faraday recalled an earlier experiment by the Frenchman Dominique Arago in which a rotating magnet caused a copper disc to spin, proving that there were ‘currents’between them. Faraday wanted to do more than make currents. He wanted to make electricity. In 1831 he put a galvanometer between the two ends of a wire wound round an iron cylinder. When he placed a magnet inside the cylinder the needle of the galvanometer twitched. When he moved the magnet in and out of the cyclinder, the wire became electrified. This was the first non-chemical electricity. Faraday then rotated a disc between the poles of a magnet and once again produced electric current.
Utilising the magnetic field of the earth, Faraday rotated a copper disc at right angles to a compass needle and the needle moved. Then he bent a single strand of wire into a rectangular shape, included a galvanometer in one side, and rotated the wire round the galvanometer. The needle swung by up to 90 degrees. As a result, on 26 December 1832, Faraday was able to record: ‘The mutual relation of electricity, magnetism and induction may be represented by three lines at right angles to each other … if electricity be determined in one line, and motion in another, magnetism will be developed in the third.’
This was the basic theory of the new electromagnetic physics. Faraday’s interest now moved to the effect of the lines of force. It appeared that the force acted, like gravity, at a distance. The curved lines of the force could be seen in the behaviour of iron filings round a magnet. Through the 1840s Faraday treated the lines merely as indications of the direction of the force in space. Then he gradually began to think of them as being part of space itself. But although the old view of bits of force impacting each other through space might seem feasible for a force that moved in straight lines, how could such an action follow a curve? The secret had to lie in the medium itself. This mysterious medium became known as the ‘aether’. In 1850 Faraday told the Royal Society: ‘Lines of magnetic force can cross space, like gravity and electricity. So space has a magnetic relation of its own, and one that we should probably find hereafter to be of the utmost importance in natural phenomena.’
Sketches from Faraday’s diary in 1831 illustrate experiments in electromagnetic force: an arrangement of magnets and coil (a), the rotation of a wire loop in the earth’s magnetic field (b) and the rotation of a copper disc between magnetic poles (c).
The effect of magnetic force on iron filings. This pattern, which reveals the curved lines of the force, was ‘fixed’by Faraday and put in his diary.
During his early tests