Georg Simon Ohm formulated the relation V = IR (Ohm's law) in the year:

Georg Simon Ohm (1789-1854) was a German physicist and mathematics teacher. In 1827, he published his book "Die galvanische Kette, mathematisch bearbeitet" ("The Galvanic Circuit Investigated Mathematically"), in which he presented the relationship between voltage, current and resistance that is now known as Ohm's law.

Ohm's law states that, at a constant temperature and other physical conditions, the electric current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends. Mathematically:

V ∝ I, or V = IR

where R is a constant called the resistance of the conductor. The SI unit of resistance is the ohm (Ω), named after him. One ohm is the resistance of a conductor through which a current of one ampere flows when a potential difference of one volt is applied across it.

A graph of V against I for an ohmic conductor, such as a metal wire at constant temperature, is a straight line passing through the origin. Materials that do not follow Ohm's law, such as semiconductors, diodes and electrolytes, are called non-ohmic conductors.

The resistance of a conductor depends on its length (directly proportional), its area of cross-section (inversely proportional), its material (resistivity) and its temperature. The resistance of metals increases with temperature.

At first, Ohm's work was not accepted by many scientists in Germany, and he lost his teaching job. Later, his work was recognised, and he was awarded the Copley Medal by the Royal Society of London in 1841. He became a professor at the University of Munich in 1852.

Other important dates in electricity:

• 1800: Alessandro Volta invented the voltaic pile, the first battery.
• 1820: Hans Christian Ørsted discovered the magnetic effect of current.
• 1831: Michael Faraday discovered electromagnetic induction.
• 1865: James Clerk Maxwell published his theory of electromagnetism.

Why not the other options?

• ✗1800 is when Volta invented the battery.
• ✗1831 is when Faraday discovered electromagnetic induction.
• ✗1865 is when Maxwell published his electromagnetic theory.