Ohm's Apparatus of 1826.
© Science Museum / Science & Society Picture Library
Reconstruction of an apparatus demonstrating the deflection of a magnetised needle turned by the current in the copper bar within the thermocouple. Georg Ohm (1789-1854), the German physicist, formulated 'Ohm's Law' which states that the current flowing through certain conductors is directly proportional to the potential difference (voltage) acros it, provided that there is no change in the physical conditions (such as temperature) of the conductor. This is now a fundamental of electrical circuit design.