Ampère table

21 x 29.8 x 51.2
Wood, copper, brass and bronze

CAT. 1851 : 1. M.III IV

Quatro apparelhos d´Ampere para as experiencias sobre as correntes voltaicas, sendo dois para as correntes circulares e no sentido horisontal, e os outros dois para as correntes dirigidas em differentes sentidos n'um plano vertical. Pertencem a estes apparelhos quatro conductores circulares, formados de laminas de cobre; diversos conductores rectilineos: um conductor circular formado por um fio de cobre; dois cylindros electrodynamicos, e um multiplicador. - Vaõ em M. III e M. IV.

Four instruments by Ampère for experiments on voltaic currents, two being for horizontal, circular currents, and the other two for currents moving in different directions on a vertical plane. These instruments have four circular conductors made of copper: several rectangular conductors: a circular conductor made from a copper wire; two electrodynamic cylinders, and a multiplier. - They are in M. III and M.IV.

It was François Arago who brought news of Oersted's discoveries to Paris, after he had witnessed them during a visit to Geneva. The members of the Academy of Sciences were initially sceptical of his report and were only convinced by his demonstration, which took place on September 11, 1819. Ampère was present at the demonstration. This event was the starting point for Ampère's research work, which was to include the use of a magnetic needle to detect an electric current. This is how a new measuring instrument, known as the galvanometer, was born. It permitted evaluation of the characteristics of an electric current in a circuit comprising a wire and a voltaic cell.

Until then, it was thought that the mechanism of the current inside the battery was separate from the current that flowed along the wire connecting the poles of the battery. Ampère concluded that the electric current within the Volta battery was the same as that in the rest of the circuit. Magnetism was nothing more than circular electric currents, and this was the theory that Ampère upheld to the day he died.

The research he carried out aimed to show that circular electric currents produced the same effects as permanent magnets. First he tried to demonstrate this effect by using a copper wire coil. When two coils were placed close together, an attraction was observed between them. If the ends of the wire, connected to the voltaic cell, are commuted, then the two coils repelled one another. Ampère published the results of his studies on September 25.

Ampère's theory that magnetism was only an effect produced by circular currents caused some difficulties. How could the permanent nature of circular currents in a magnet be explained? Ampère, following a suggestion of Fresnel, said that they should be observed in the individual "molecules" of the magnet. To test this theory Ampère and Arago, independently, carried out some experiments.

Arago observed that an iron needle placed inside a copper wire coil was magnetised when the ends of the wire were connected to a battery. This was the first electromagnet ever made. According to Ampère's theory, the circular current in a circuit made of a copper coil and a battery induced a circular current in the iron needle. Two alternatives were possible, he believed: first, the circular current was established over the whole length of the iron; second, the circular current was taking place inside the constituent "molecules" of the needle.

Ampère's experiment to determine which of the two alternatives was correct consisted of suspending a copper ring inside a coil made of insulated copper wire in such a way that the axis of the ring and the coil coincided. If the magnetisation observed in the needle used by Arago was the result of a circular current around its axis, then magnetisation should also be observed in the copper ring, signifying the induction of a circular current along it. However, when a magnet was brought close to the ring, Ampère did not observe an interaction between the two. Ampère thus came to accept the validity of the theory that the circular currents causing the magnetism in the needle used in Arago's experiments were currents established within the "molecules".

Countless scientific and teaching instruments were subsquently developed to investigate and teach the phenomena of interaction between electrical currents. Ampère created a series of devices in a wide variety of geometric shapes to demonstrate the behaviour of conductors when an electrical current passes through them. The instrument presented here is one of them.

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