01Executive summary
Nine milestones over about a century and a half, at the end of which a complete engineering discipline could be built from results none of whose discoverers were engineers.
The Leyden jar stored charge before anyone could say what charge was. Coulomb made the subject quantitative in 1785. Volta produced a continuous current in 1800. Oersted linked electricity to magnetism in 1820, Ohm gave the circuit its governing relation in 1826, and Faraday found induction in 1831. Maxwell unified the field in 1865, Hertz confirmed his prediction in 1887, and J. J. Thomson identified the carrier in 1897 — long after the industry using it was established.
02Static electricity: a subject without a use
The Greeks knew rubbed amber attracts light objects, and for a long time that attraction was confused with the quite different attraction of a lodestone for iron. Cardan distinguished them, and in 1600 Gilbert set the difference out in De magnete: amber attracts small light bodies but not iron, a lodestone attracts iron only. Gilbert found glass, gems, sulphur, sealing wax and resin behave like amber, and coined a Latin term for them from the Greek word for amber. The English word electricity first appears in 1646.
Two results from this period became engineering components. Gray showed in 1731–32 that some substances conduct and others insulate. And in 1745–46 van Musschenbroek at Leyden and von Kleist in Pomerania independently found that charge can be stored on two conducting surfaces separated by an insulator.
Von Kleist had the result first and described it so poorly that most readers could not reproduce the experiment, which is why the device is named after Leyden. Reproducibility, not priority, decided the attribution — and that is arguably the correct outcome. As for the effect itself, van Musschenbroek's first discharge was violent enough that he opened his report with a warning never to attempt the experiment. The capacitor entered engineering as a device that hurt people before it was understood at all.
Theory lagged behind. Du Fay proposed in 1733 that there are two electricities, vitreous and resinous, each repelling its own kind and attracting the other. Franklin replaced this in 1747 with a one-fluid theory that proved useful and productive for over a century. Neither was right, and both supported real experimental progress. Then Coulomb in 1785 verified Priestley's inference that the force between two small charged spheres varies inversely with the square of their separation. That is the first quantitative law in the subject and the point at which electricity became something an engineer could eventually calculate with.
03Volta: a steady current, from an argument about frogs
Galvani found in 1786 that a frog's leg convulsed when he touched muscle and nerve with a conductor made half of zinc and half of copper, and not when the conductor was a single metal. Two explanations were available: the dissimilar metals were producing electricity by contact, or the tissue was producing it with the metals merely conducting. Galvani chose the second and coined "animal electricity".
Volta initially accepted this and then found he could not explain all his results with it. Suspecting the metals were the source, he replaced the frog's leg with a sensitive electroscope of his own design and detected a charge when two dissimilar metals were brought into contact with no biological material present at all. Pushing further he arrived at the pile, announced in 1800.
Static electricity can be stored and discharged. It cannot be made to flow steadily, so it can do essentially no work. Volta's pile made a continuous current available for the first time, and every electrical development after it — electromagnetism, the telegraph, electroplating, lighting, motors, power — requires a steady source rather than a stored charge. It is also a good example of a general method: Galvani's experiment contained two variables, and Volta made progress by removing one of them rather than by arguing about which explanation was more plausible.
04Electricity and magnetism become one subject
Attempts before 1800 to connect electricity and magnetism failed, and the reason is instructive: there is no connection between static electricity and magnetism, only between a current and magnetism. Nobody could find the link because the current did not exist yet.
- Oersted, 1820A current in a wire above a compass swings the needle nearly at right angles to the wire. Electricity and magnetism are the same subject.
- Ampere, 1820–22Within two weeks of the announcement: a coil behaves as a magnet, two coils attract and repel like magnets with no iron present. He also defines potential as distinct from current, and establishes electrodynamics by 1822.
- Ohm, 1826Current is proportional to potential difference and inversely proportional to resistance. Circuits become calculable.
- Faraday, 1831A changing magnetic field induces a current in a conductor. Motor, generator, induction coil and transformer all follow from one result.
- Maxwell, 1865Field equations relating conductivity, dielectric constant and permeability to electric and magnetic fields and mechanical force — and the prediction of waves travelling at the speed of light.
- Hertz, 1887Those waves produced and detected with a spark oscillator and a ring detector, shown to reflect and refract, and measured at the speed of light.
Faraday and Henry
Joseph Henry discovered induction before Faraday, working independently, and published later. Priority conventionally goes to Faraday because he published first, in April 1832. Both facts should be stated together.
Two different Thomsons
Sir William Thomson, Lord Kelvin, was largely responsible for the success of the Atlantic cable. J. J. Thomson identified the electron in 1897. They are different people and are frequently confused.
Faraday's 1831 work deserves the emphasis it gets. He was convinced that if electricity could produce magnetism then magnetism must produce electricity, and looked for it for about six years before finding that the current appears only while the field is changing. That qualification is the whole content of the result, and it is why a generator must move and a transformer must run on alternating current. A great deal of subsequent engineering is contained in one word.
05The carrier identified, last
Cathode rays were found by Plücker in 1859. Hittorf showed a decade later that they travel in straight lines and are deflected by a transverse magnetic field. Crookes showed by 1879 that they carry momentum and energy. And in 1897 J. J. Thomson demonstrated that they are negatively charged particles — the first physical evidence that such particles exist.
Thomson's result also explained something Edison had found and not pursued. In 1883 Edison observed that a small current would pass from the hot carbon filament of an evacuated lamp to a separate electrode when that electrode was charged positively, and not when charged negatively. Preece and Fleming studied the effect in detail through the 1880s and 1890s. In 1903 Thomson showed that electrons carry that current, and the phenomenon became the basis of the thermionic valve and of communications electronics.
Telegraphy was a mature industry by 1866. Central stations were selling electricity by 1882. The alternating-current system was in commercial operation by 1886. The particle carrying the current in all of it was identified in 1897. An engineering discipline can be built, industrialised and made profitable on relationships between measurable quantities without knowing the mechanism underneath — provided the relationships are correct and quantitative. This is worth holding onto, because the reverse error is common: waiting for a mechanistic explanation before acting on a reliable empirical relationship.
06Takeaways for current practice
- Remove a variable rather than arguing about interpretation. Volta replaced the frog's leg with an electroscope and the dispute ended.
- A wrong theory can still be productive. Both the two-fluid and one-fluid accounts were wrong and both supported real experimental progress.
- Reproducibility decides attribution, and should. Von Kleist was first and unreadable.
- Watch the qualifying word. Induction occurs only while the field is changing, and that clause is the entire design basis of generators and transformers.
- You do not need the mechanism to engineer with the relationship. A whole industry preceded the identification of the electron.