The discovery of the electric current

The discovery of the electric current

ELSEVIER Physica B 204 (1995) 367 369 The discovery of the electric current Piero Cotti Technikum Winterthur. Ingenieurschule, CH 8401 Winterthur, S...

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ELSEVIER

Physica B 204 (1995) 367 369

The discovery of the electric current Piero Cotti Technikum Winterthur. Ingenieurschule, CH 8401 Winterthur, Switzerland

Abstract The first battery, the so called voltaic pile, turns out to be the only and hidden entrance to the world of electrodynamics. It was not until 20 years after Alessandro Volta's discovery that the realisation came that the sensational novelty of the voltaic pile was not the permanent voltage source but the current source. This was not to be expected, and had, therefore, not been searched for specifically, but, rather had been found through a great deal of luck and coincidence in experimentation.

When Alessandro Volta (1745-1827) wrote his famous letter to the president of the Royal Society, Sir Joseph Banks, on 20 March 1800 on how a permanent voltage source can be constructed by a suitable arrangement of various metals and salt water, he did not realise that through this he had opened the doors onto a huge new world of science and technology, electrodynamics, the world of current electricity. In the year 1800, the concept of electric current was not yet known. Following the discovery of the voltaic pile, the talk was of'electrical conflict' in a metal wire attached to a voltaic pile, and it was Andr6 Marie Amp6re who realised in 1820 that electric current means charge transport. Sources of electric current were not looked for because it could not have been foreseen that a moving charge generates magnetism. Electrodynamics was discovered by chance and with a great deal of luck! It began in 1756 with Leopoldo Marc-Antonio Caldani (1725-1813), who observed that frogs' legs gave out hefty spasms when they came into contact with frictional electrostatic generators. In September 1786, Luigi Aloisio Galvani (1737-1798) succeeded in making a sensational discovery: a frog's leg gave out a spasm even when there was no electrostatic generator on for miles. We understand today what happened to Galvani: he touched a freshly prepared frog's leg with two different metal instruments by accident. As a frog's leg contains

electrolytic liquids, Galvani had the first electric battery in his hands, but as a frog's leg also contains nerves, it was both source and detector of a new phenomenon. Even here coincidence had a part to play! Galvani believed he was on the track of the secret life-force. It was Volta who realised that it was not life that was the reason behind this new phenomenon, but the contact between different materials. But even the great Alessandro Volta had got it wrong, as we can conclude today from our elevated positions. Volta's experiments and reflections are a handsome example of how a false theory can lead to experimental success. We now want to try and examine this historical detail somewhat more closely: After 1792, Volta searched for the contact voltage between different metals. In 1795, he succeeded in building a sensitive static electrometer, in which he made use of a variable capacity (Fig. 1). Thus, he had a very sophisticated measuring device at his disposal, with which voltages of around one volt could be measured. In 1799, he carried out the first measurements of contact voltage between metals, a phenomenon which is only measurable when the contact is broken (Fig. 2). He concluded that in a closed circuit of various metals, the sum of the contact voltage is zero (the so-called Volta fundamental law, by today's interpretation, because the Fermi-level has to be the same everywhere). So Volta had

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P. Cotti/ Physica B 204 (1995) 367 369

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to realise that a permanent power source cannot be constructed using only different metals. In 1799, he questioned the matter of contact voltage between a metal and a liquid, which he had also measured on breaking contact (Fig. 3). Thus, his experiments showed that the contact voltage between metals and liquids is apparently zero. Here, Volta displayed a flaw in his reasoning. In his experimental set-up, a slight liquid film clung to the metal upon breaking contact. Therefore, he had not separated metal from liquid, rather only liquid from liquid. Then Volta saw how a permanent power source could be constructed (Fig. 4). He successfully connected unidirectional silver to zinc contacts in which he compensated (according to his theory) for the opposing zinc to silver contact by a layer of liquid. Volta believed until his death in 1827; as well as most of the physicists of his time (with the exception of the unfortunate Johann Wilhelm Ritter (1776-1810)), that the reason behind the voltaic pile's function was the metal contact. It was not until 1836 that Auguste de Larive (1801-1873) delivered the modern theory of the voltaic pile: when metal electrodes are immersed in liquids, a tendency, different from metal to metal, exists for the metal atoms to diffuse as ions in

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the liquid, thereby resulting in a measurable voltage between electrode and liquid. The voltaic pile meant that after 1800 the doors to the world of electrodynamics stood open and yet another 20 years passed before Hans Christian Oersted (1777-1851) succeeded in making the decisive breakthrough. On 20 July 1820 Oersted's four-sided publication in Latin appeared. An electric current influences a compass needle. Electricity and magnetism join together. It was not until the beginning of the 20th century that Albert Einstein (1879 1955) succeeded in demonstrating that magnetism can be interpreted as a Lorentz-transformafion of the

P. Cotti/ Physica B 204 (1995) 367-369 electric coulomb force, so electricity and magnetism actually have the same cause. Coming back to Oersted: A connection between electricity and magnetism had long been sought after, especially after William Gilbert's (1544-1603) iegandary book on magnetism, which appeared in 1600 and which, in the preface, is dedicated specifically to those who do not only read but also experiment... Gilbert states that marvellous distant effects similar to magnetism are shown not only by amber (electron in Greek) but that wax, glass, sulphur and precious stones can also be electrified by rubbing. Thus, an isolated phenomenon became a new science. Gilbert thrust open the doors to the world of electrostatics. Oersted looked for a bridge between static electricity and magnetism by moving a compass needle close to the pole on a currentless voltaic pile, but was, of course, unable to observe any effect. In 1820, Oersted demonstrated 'electrical conflict' during a lecture: by the heating of a wire which was attached to the poles on a voltaic pile. A compass needle which had, by chance, been placed on the table showed a slight rotation. So here too, coincidence had a very important part to play in the history of science. On the other hand, one possibly also thinks of the discovery of natural radioactivity by Antoine Henri Becquerel (1852-1908), who sought to generate X-rays from sunlight fluorescence and discovered on 1 March 1896 that uranium gives out permanent radiation even without sunlight. How else could natural radioactivity have been discovered? Thus, however, the doors to the world of nuclear physics had been opened. After Oersted's discovery, purposeful experimentation could be continued. Andr6 Marie Amprre (1775-1836) made the next great step. In a burst of creative energy during the week of 11-18 September 1820, he succeeded in making four discoveries of very great importance. Amprre was the first to discover that the essence of electric current is charge transport. He traced the current to current interaction and postulated that the current is always the cause of the magnetism, and that permanent mysterious circulating currents always flow in a permanent magnet, which was proved by experiment nearly 100 years later by Einstein and Johannes Wander de Haas. Einstein and de Haas's study is entitled "Experimental Proof of Amprre's Molecular Currents". The study is handsome proof that, as a precaution, an experiment

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should be repeated when it produces the expected results... Einstein and de Haas were wrong by a factor two, because they were thinking of circulating electrons and did not yet known anything of electron spin. Amprre's fourth discovery was an experimental trick which appears so trivial today that its ingenuity is hardly ever acknowledged: Amprre invented the coil. Ever since then, it has been possible for science and technology to produce strong maaneticfields (for instance the strongest static magnetic fields in the world made in Grenoble). Most especially, with coils, it was finally possible to manufacture strongly magnetised compass needle (William Gilbert's dream). The next decisive steps in the world of electrodynamics were made by the amiable Michael Faraday (1791-1867). If a current influences a compass needle, then a permanent magnet must also influence the current (actio = reactio). On 4 September 1821, Faraday built the first very slow-turning "electric motor". According to his young wife, his eyes shone with happiness while he did this... As current generates magnetism, according to Oersted and Amp&e, does it not also work the other way around, to generate a current from magnetism? Faraday spent 11 years looking for such a device and also kept a detailed diary of his search. On that memorable day, 29 August 1831, he discovered the electromagnetic induction, when a primary current in a ring-shaped transformer set-up was switched on and off, this induced a current surge in a secondary coil, which could be traced by a compass needle. What, for Faraday, was a barely traceable, weak effect, is today an indispensable instrument in technology. From bicycle dynamos to nuclear power stations, the aim is always to get a coil to move in a magnetic field. A sarcastic comment would be appropriate at this point: In view of the great historical importance of coincidence in experimentation, perhaps more coincidental experiments should be planned and financed these days.

References [1] Mueller-Pouillets, Lehrbuch der Physik, Vol. IV, pp. 314-351 (Vieweg, Braunschweig, 1909). [2] F. Fraunberger, Illustrierte Geschichte der Elektrizit/it (Aulis Verlag Deubner, K61n, 1985). [3] F. Sattelberg, Vom Elektron zur Elektronik (Elitera, Berlin, 1971).