André Marie Ampère Biography

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World of Physics on André Marie Ampère

André Ampère was born on January 22, 1775,in Lyon, France. He was the son of a well-to-do merchant. He mastered advanced mathematics by the age of twelve and learned Latin so he could read the works of the learned scientists.

Ampère was interested in a variety of subjects, including psychology, philosophy, astronomy, physics, and chemistry. He studied the nature of chlorine and iodine, but the credit went to Humphry Davy . Ampère analyzed Boyle's law and, in 1814, studied the molecular makeup of gases, independently coming up with what has become known as Avogadro's number. He spent most of his life teaching and giving lectures, primarily in mathematics and chemistry, at various institutions.

In 1819 a discovery occurred that had a profound effect on science in general, and Ampère in particular: Danish physicist Hans Christian Ørsted placed a magnetized needle next to a wire carrying an electric current and saw the needle deflect. This was the first indication that electricity generated a magnetic field, linking the two forces. Ørsted published the results of his experiment in 1820; electromagnetism subsequently became Ampère's life work.

Ampère and his countryman Dominique-Françios Arago became leaders in investigating the link. Within a week of learning of Ørsted's work, Ampère had devised the right-hand screw rule to show the direction in which a magnetic needle will be deflected. This introduced the concept of lines of force, which figured prominently in the work of Michael Faraday. After Ørsted's magnetic needle was deflected by an electric current, Ampère realized that it could be used to measure the flow of that current if placed next to a graduated scale.

The only problem with the right-hand screw rule was in determining the direction in which the current flowed. Benjamin Franklin, who pioneered the concept of positive and negative electricity, had made the assumption that current moved from the abundant pole (positive) to the deficient pole (negative). That seemed logical to Ampère, and he used it to determine the direction of current flow. Alas, Franklin's assumption was incorrect; fortunately, because of consistent reversal, this made no difference to Ampère's results. To differentiate between his flowing current and the static charge with which Franklin had worked, Ampère coined the term electrostatics.

Continuing to experiment with electricity, Ampère discovered that two wires, placed end to end, attracted each other when they were carrying current flowing in the same direction, and repelled each other if the current flowed in opposite directions. If one wire was allowed to rotate around the other, it would move through a semicircle and stop when its current matched the flow of the current in the other wire.

What would happen if the wire was circular, Ampère wondered. In theory, the electric current flowing through a wire helix, or coil, would set up a magnetic field and create an electro-magnet that would act like a bar magnet. Ampère dubbed the wire helix a solenoid, a device with which Joseph Henry experimented, leading to a revolution in communications.

In 1823 Ampère published a mathematical theory that suggested magnetism was electricity in motion. He showed that a magnet's properties could be explained if one assumed there were many tiny electric currents circling around within it. Numerous scientists refused to accept this concept, which was far ahead of its time; 60 years later the negatively charged electron was discovered.

In 1827 Ampère published his Theory of Electrodynamic Phenomena, in which he established precise mathematical formulations of electromagnetism. Ampère's Law relates the electromotive force that is created by the currents in two parallel conductors to the product of their currents and the distance between the conductors.

Ampère died in Marseille, France, on June 10, 1836. His memory was honored in 1883 when William Thomson (Lord Kelvin) proposed measuring the quantity of current passing a given point at a given time in amperes. It was very appropriate; Ampère had been the first to distinguish between the rate in which current passed and the force (measured in volts, after Alessandro Volta) that pushes it.