In 1820, a French physicist invented an astatic needle which canceled out the Earth's magnetic field, enabling the creation of the sensitive Galvanometer

André-Marie Ampère was a French physicist whose work helped establish the study of electrodynamics, which later became part of the broader field of electromagnetism. He was born in Lyon on January 20, 1775, and died in Marseille on June 10, 1836. His work connected electricity, magnetic fields and mechanical forces. In 1820, Ampère developed an astatic needle arrangement that reduced the effect of the Earth's magnetic field. This made it possible to build a more sensitive galvanometer for detecting electric currents. His work also included the solenoid, the study of forces between current-carrying wires, Ampère's circuit law and an early electrical telegraph design.

The 1820 invention and how the astatic needle worked

In 1820, a French physicist invented an astatic needle which canceled out the Earth's magnetic field, enabling the creation of the sensitive Galvanometer. The invention came during a period when scientists were trying to understand the connection between electricity and magnetism.


The Danish physicist Hans Christian Ørsted had shown that an electric current could deflect a magnetic needle. François Arago later demonstrated this discovery before members of the French Academy of Sciences.

Ampère was already interested in mathematics, physics and other areas of science. He began studying the relationship between electricity and magnetism after learning about Ørsted's experiment.

The astatic needle was based on a pair of magnetic needles placed in opposite directions. The arrangement reduced the effect of the Earth's magnetic field. This was important because the Earth's magnetic field could interfere with measurements. Reducing this effect allowed scientists to detect weaker electrical currents.

The arrangement helped with the development of a sensitive galvanometer. A galvanometer is an instrument used to detect and measure electric current. Ampère's work therefore connected an experimental device with the wider study of electricity and magnetism.

Ampère's work on electric currents

Ampère did not limit his research to the astatic needle. He used experiments and mathematics to study how electric currents behave. One of his main findings involved two parallel wires carrying electric currents. He found that the wires could attract or repel each other depending on the direction of the currents.

The basic pattern was:

• Parallel currents flowing in the same direction attract each other.
• Parallel currents flowing in opposite directions repel each other.
• The force depends on the currents and the arrangement of the wires.
• The length of the conductors also affects the interaction.

These observations became part of what is known as Ampère's force law. The work provided a mathematical way to describe the force produced between current-carrying conductors. It became part of the foundation of electrodynamics. Ampère also connected his findings with earlier work on magnetic forces, including the work of Charles-Augustin de Coulomb.

The solenoid and magnetic effects

Ampère also worked on the solenoid. A solenoid is a cylindrical coil of wire. When an electric current passes through the coil, it produces a magnetic field. The device helped demonstrate the relationship between electricity and magnetism.

Ampère's research showed that electric currents could produce magnetic effects. This supported his wider effort to explain magnetic phenomena through electric currents. The solenoid later became important in electrical devices and electromagnetic systems. The basic principle remains relevant in equipment that uses coils and magnetic fields.

His early electrical telegraph idea

Ampère also proposed an early design for an electrical telegraph. His concept used multiple wires and magnetic needles. Each wire could be associated with a particular needle. When an electrical signal passed through a wire, it could produce movement in the corresponding needle. This created a method for sending information through electrical signals.

The design was an early attempt to use electricity for communication. Later telegraph systems developed the idea further and used electrical signals to transmit messages over distances. Ampère's proposal shows that his work was not limited to theoretical questions. He also considered how electromagnetic effects could be used in practical devices.

Ampère's circuit law

Another major part of his work was the relationship between electric current and magnetic fields. Ampère developed a mathematical description of the magnetic field produced by electric currents. This became known as Ampère's circuit law.

In simple terms, the law describes how electric current produces a magnetic field around a conductor. The principle became part of the mathematical framework used to study electromagnetism. His work helped move the study of electricity and magnetism from individual observations toward mathematical descriptions.

Separating current and voltage

Ampère also helped establish a distinction between electric current and voltage. Electric current refers to the movement of electric charge through a conductor. Voltage refers to the electrical potential difference that drives current through a circuit. The distinction became important for understanding electrical circuits. Later developments in electrical science built on these concepts and established the terminology and measurement systems used in electrical engineering.

Ampère's work with chemical elements

Ampère also studied chemistry. In 1811, he suggested that hydrofluoric acid contained an element that was distinct from chlorine. He proposed the name fluorine. He did not isolate fluorine himself. Fluorine was later identified and isolated by other scientists. Ampère also attempted to classify chemical elements according to their properties.

In 1816, he grouped elements based on similarities in their characteristics. His classification did not become the modern periodic table, but it showed his interest in finding patterns among chemical elements. His scientific work therefore covered several fields.

His early education and family life

Ampère was born in Lyon to Jean-Jacques Ampère and his wife. His father was a merchant and followed the educational ideas associated with Jean-Jacques Rousseau. Ampère did not follow a conventional school education during his childhood. His father allowed him to study in the family's library.

The library contained scientific and mathematical books. Ampère began teaching himself advanced mathematics at about 12 years of age. He also read works such as Georges-Louis Leclerc, Comte de Buffon's Histoire naturelle and the Encyclopédie associated with Denis Diderot and Jean Le Rond d'Alembert.

His mother introduced him to the Catholic faith. The French Revolution also affected his family. His father became a justice of the peace under the revolutionary government. After the Jacobins gained control, he was arrested and executed by guillotine on November 24, 1793. This event affected Ampère during his youth.

Ampère's academic career

Ampère began working as a mathematics teacher in 1799. In 1802, he became a professor of physics and chemistry at the École Centrale in Bourg-en-Bresse. During this period, he also worked on mathematics and published a study on probability and games. After his wife's death in July 1803, Ampère moved to Paris.

He began tutoring at the École Polytechnique in 1804. In 1809, he became a professor of mathematics there. He remained connected with the institution until 1828. He also taught philosophy and astronomy at the University of Paris in 1819 and 1820. In 1824, he became a professor of experimental physics at the Collège de France. In 1814, he joined the mathematics section of the Institut Impérial.

The founding of electrodynamics

Ampère's research after 1820 became the central part of his scientific career. He developed a mathematical and physical explanation for electromagnetic effects. He proposed the idea of an "electrodynamic molecule" as part of his attempt to explain electricity and magnetism. This was an early theoretical concept and should not be treated as the modern electron.

Ampère later published his major work on electrodynamics. His treatise, Mémoire sur la théorie mathématique des phénomènes électrodynamiques uniquement déduite de l'experience, was published in 1827. The work presented his mathematical theory of electrodynamic phenomena and became an important text in the development of the field. Ampère also gave the new science the name electrodynamics. His research helped establish the study of electromagnetic forces as a field of scientific investigation.

Why Ampère's name is still used?

Ampère's contribution was later recognized through the international system of electrical measurements. In 1881, an international convention established the ampere as a standard unit of electrical current. The unit was named after André-Marie Ampère.

Other units adopted during the same period included the coulomb, volt, ohm and watt, named after scientists including Charles-Augustin de Coulomb, Alessandro Volta, Georg Ohm and James Watt. The ampere remains part of the International System of Units.

What the astatic needle meant for science?

The astatic needle was one part of a larger period of change in electrical science. The device reduced the influence of the Earth's magnetic field on measurements. That mattered because researchers needed instruments that could respond to small electrical currents without interference from the surrounding magnetic field.

The development of the sensitive galvanometer provided a way to detect such currents. Ampère's work then moved beyond instruments and experiments. He developed mathematical laws to describe the forces produced by electric currents and the magnetic fields associated with them. His research helped establish principles that later became part of electromagnetism.

Ampère's final years

Ampère's 1827 treatise marked the end of the main period of his original scientific work. His health later declined. He continued his academic responsibilities and was carrying out a university inspection when he died in Marseille on June 10, 1836. He was 61 years old.

His scientific work continued to influence electrical science after his death. The astatic needle, galvanometer, solenoid, Ampère's force law and Ampère's circuit law represent different parts of his contribution. Together, they show how Ampère used experiments and mathematics to study the relationship between electricity and magnetism.