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Natural Science - Year II

Unit 51: Early Theories of Electric Current and Magnetism

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History Weblecture for Unit 51


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History Lecture for Unit 51: Faraday, Electric Current, and Fields

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Outline/Summary

Current Electricity

Sir Humphrey Davy's and Alessandro Volta's battery experiments all involved building electrical circuits, that is, creating pathways for the electrons to flow from the positive terminal through some conducting material (like fluid in a pipeline) back to the negative terminal of the battery. If the circuit were broken by disconnecting the wires or draining the conducting acid from the battery, no electricity would flow. The amount of current could be estimated by its ability to drive certain kinds of chemical reactions, and Davy used it to decompose many compounds into their separate elements. Such measurements, however, were not exact enough, nor did the simple circuits Davy and Volta used provide enough control, to help them determine all the forces, currents, and electric energy levels involved. For that to happen, the experimenters needed another way of looking at electrical phenomena.

Naturphilosophie and Hans Christian Oersted

You may have seen a book called Physics for Poets, which is a non-mathematical introduction to physical ideas for people who don't consider themselves scientists. In the late eighteenth century, however, there was a movement in German philosophy which might well have been called Poetry for Physicists. Its leader was Friedrich Schelling, a German philosopher who had been influenced by the romantic poets Goethe and Schiller. In contrast to, or perhaps even in direct opposition to, the mechanical and deterministic world view which was the outgrowth of Newton and Leibniz's theories, Schelling proposed a more organic view of nature, one in which all forces, attractive and repulsive, were expressions of matter. In his opinion, the fundamental aim of science was to investigate and interpret nature as a unified entity, and he believed that all forces — mechanical, electrical, magnetic, chemical, and "vital" (whatever it is that distinguishes living from non-living matter) — were manifestations of a single force.

Schelling's approach attracted a number of poets; it also attracted a physics professor named Hans Christian Oersted, who had noticed that his compass needle fluctuated during thunderstorms. This made him wonder whether or not magnetism and electricity might be related, as Schelling thought. He built a circuit using a battery and connecting wire, then placed a magnet above the wire and below it. He discovered that the magnetic needle would point in opposite directions, depending on its position. It was obvious that the current in the wire and the magnetic force in the compass were interfering with each other.

Andre Marie Ampere

Oersted's discoveries were announced to the French Académie des Sciences in Paris in September of 1820. A week later, André-Marie Ampère announced that he had detected a force similar to magnetic attraction between two wires carrying current in the same direction; if the current flowed in opposite directions, the wires repelled one another. This phenomenon at last gave the investigators a way of measuring the amount of current in the wires directly, as a function of the force of attraction or repulsion observed in the wires. Ampère was careful to make a distinction between electrical potential energy, which he could always measure in a battery even when it wasn't hooked up to a circuit, and the current which would flow only in a completed circuit.

Read about André-Marie Ampère at the St. Andrew's Mathematics site.

  • Who influenced Ampère?
  • What contributions did he make to the theory of light?
  • What contributions did he make to the theory of chemistry?
  • How did Ampère find out about Oersted's observations of compasses and lightning?

Using Ampère's distinction, Georg Simon Ohm measured the relationship between the electric potential energy of the battery (V, measured in volts) and the current in a given conductor. He found that conductors made of different materials carried different amounts of current even when the battery was supplying a constant amount of voltage. The relationship between the current flow I and amount of electrical energy V supplied to the circuit could be summed up in a neat little equation that depended on the resistance R in the material:

V = IR

This fundamental relationship is called Ohm's law, and it establishes the relationship between Volts of electrical potential (V), Ampères of current (flow of electrons per second), and Ohms of resistance.

Michael Faraday

One of the many members of the general public who attended Sir Humphrey Davy's popular lectures on electrical phenomena was a bookbinder named Michael Faraday. Faraday's formal education ended when he was 13, but his position in the book bindery gave him the opportunity to read about many subjects. He took notes at Davy's lectures, studied, and eventually applied to Davy for a job as an assistant. Davy hired at less money than Faraday made binding books, but took him on a grand tour of Europe, where Faraday met Ampère and Volta.

Inspired and encouraged by these contacts, Faraday continued to train himself. He was driven not only by his own curiosity but also by an abiding conviction in the unity of the created order, a conviction which grew out of his Christian faith. Despite frequent illness (probably from the same mercury poisoning which appears to have caused Davy and Newton problems), he managed to not only put in long hours at his laboratory, but also to give public lectures and to support his local church.

Today Faraday is universally acknowledged as one of the finest experimentalists in the history of science. His researches included studies of chemical compounds, electromagnetic phenomena, the behavior of gases, and the nature of light. He invented the first electric motor (in 1821), in which current flowing through a coil of wire caused a magnet to move. Having shown that electricity could cause magnets to move, he then spent the next decade trying to produce the reverse phenomenon, and generate electrical current in a coil by moving a magnet. Finally, in October of 1831, he was able to successfully produce electromagnetic induction; his invention is the electric generator or dynamo. Faraday's two inventions, the electric motor and the electric generator, are fundamental necessities in our modern dependence on electrical power. He then went on to determine how much electricity was necessary to separate elements during electrolysis, the process of passing current through solutions which Davy had used to isolate sodium, calcium, barium, and other substances from their compound forms. In the 1840s, Faraday showed that polarized light passing through a magnetic field would change its direction of polarization, but he lacked the mathematical theory to explain why this worked.

Read more about Michael Faraday at the St. Andrew's Mathematics site.

  • How did Faraday receive his training in science? Who were major influences on his way of thinking?
  • What contributions did he make to the theory of electricity?
  • What contributions did he make to the theory of chemistry?
  • What contributions did he make to methods of experimentation?

Faraday tried to develop a way of thinking about electricity and magnetism that would be more useful than the limited concepts of "electrical fluids", which could not explain how magnetic and electrical phenomena interacted. Since he was not a mathematician, he tended to think in pictures, and it is to Faraday that we owe the idea of force fields, which we now use to explain how all kinds of forces—electromagnetic, gravitational, and nuclear—work.

The picture at the top of our Natural Science pages shows Faraday lecturing at the Royal Institute to his fellow scientists. Faraday loved teaching, and though he had no children of his own, he devoted a great part of his later years to Lectures at the Royal Institute especially designed for "the Juvenile Auditory" -- young people of 10-14 years of age. During these lectures, no one over 14 was allowed to sit in the first five rows of the lecture hall, so that the young people could see Faraday bounce across the stage, performing scientific demonstrations and experiments very much like Bill Nye does now.

Joseph Henry

In another example of simultaneous discovery, the American Joseph Henry also produced electromagnetic induction, but did not publish his work until after Faraday's reports had appeared. Like Faraday, Henry was a poor but hard-working young man, whose Christian faith led him to continue his own education and put himself through academy training. He became a teacher of mathematics and science at his own school, and eventually, after making a number of inventions and discoveries in the field of electromagnetism, the first director of the Smithsonian Institution.

Henry experimented with fluctuating current, turning the current on and off, and discovered that changing the current in a coil could cause another current in the same coil—self-induction. The unit of induction is now called the henry; it measures a difference of one volt of electric potential induced by a change of one amp per second in the current flow. [The Russian scientist Heinrich Lenz noted that such an induced current always opposes the change which produced it.] On the basis of these discoveries, Henry invented a practical working electric telegraph (prior inventions did not work over long distances), as well as the electromagnetic relay and the electric transformer, which are used today to control current flow. Henry reported his results to Faraday, and they continued to consult one another on electromagnetic induction and oscillating currents, even meeting during Henry's trip to England in 1837.

Neither Faraday nor Henry marketed their inventions; having demonstrated the possibilities, they moved on to other researches. It was left to the American Samuel Morse to construct a practical telegraph, and to the French inventor Hippolyte Pixii to make the first practical generator.

Study/Discussion Questions:

Further Study/On Your Own