1820

## Ørsted's compass

Danish physicist and chemist Hans Christian Ørsted notices during one of his lectures that the magnetic needle of a compass aligns itself perpendicularly to a current-carrying wire, suggesting a relationship between electricity and magnetism. Unlike Gian Domenico Romagnosi’s discovery of the same link between electricity and magnetism almost two decades earlier, Ørsted's announcement of the event would send shockwaves through the scientific community and lead to a flurry of new experimentation.

1820

## Ampère's wires

Mathematician André-Marie Ampère, only a week after Ørsted’s discovery was published, begins to develop a theory to explain the phenomenon and demonstrates that parallel wires with currents flowing through them attract each other when the currents flow in the same direction, but repel each other if they flow in opposite directions.

1820

## Arago's iron filings

Building on the work of Ørsted, French physicist François Arago finds that unmagnetized iron filings orient themselves in a circle around a copper wire through which an electric current flows as if it were a magnet, but disperse when the current is stopped.

1820

## Schweigger’s multiplier

German mathematician and physicist Johann Schweigger builds what he terms a multiplier that could greatly amplify the magnetism of an electrical circuit. Schweigger’s multiplier became the first accurate device capable of detecting and measuring very small amounts of electricity, eventually coming to be known as the galvanometer.

1820

## Biot-Savart law

French physicists Jean-Baptiste Biot and Félix Savart formulate what is now known as the Biot-Savart law, which can be used to calculate the magnetic field at a given distance from the electric current that is the source of the field.

1821

## Faraday's motor

Michael Faraday, a former bookbinder who was apprenticed in science under Humphry Davy, plots the magnetic field around a conductor and repeats Ørsted’s experiments in his laboratory at the Royal Institution. He discovers that electricity can produce rotary motion, leading him to build one of the first primitive electric motors.

1822

## Seebeck effect

German physicist Thomas Johann Seebeck discovers that a current flows through a circuit of dissimilar conducting materials if there is a difference in temperature between the materials. This thermoelectric effect is now known as the Seebeck effect.

1822

## Barlow's wheel

Peter Barlow, an English mathematician and engineer, demonstrates an early version of the electric motor that is commonly referred to as Barlow’s wheel.

1822

## Ampère's law

André-Marie Ampère, a French scientist, formulates his fundamental law for the relationship between a magnetic field and the current that is its source, similar to the Biot-Savart law but presented in a more sophisticated form utilizing the language of calculus.

1824

## Magnetic scalar potential

French scientist Siméon-Denis Poisson introduces the concept of the magnetic scalar potential.

1824

## Magnetic rotation

French scientist and politician François Arago discovers magnetic rotation, which occurs when a magnetic needle suspended over a copper disk rotates when the disc is spun.

1825

## Practical electromagnet

English engineer William Sturgeon develops and exhibits the first practical electromagnet, which is strong enough to support 20 times its own weight.

1827

## Improved electromagnets

In his first paper on magnetism, Joseph Henry, an American mathematics professor, describes several improvements he made to electromagnets and other devices used in electromagnetic demonstrations to produce more pronounced effects.

1827

## Ohm's law

German physicist Georg Simon Ohm publishes

*The Galvanic Circuit Investigated Mathematically*. The treatise contains an account of his electromagnetic theories and includes all of the components of Ohm’s law.1828

## Green's theorem

British mathematician and physicist George Green extends the electric and magnetic calculations of Siméon-Denis Poisson, introduces the term potential, and explicates what is now known as Green’s theorem in his

*Essay on the Application of Mathematical Analysis to the Theory of Electricity and Magnetism*.1828

## Dyar's telegraph

American entomologist Harrison Gray Dyar constructs a telegraph in which an electric signal is recorded through chemical means as a stain on moistened litmus paper caused by the decomposition of nitric acid.