Electrical Conductivity

The early studies of electrical conduction in metals were done in the eighteenth and early nineteenth centuries. Benjamin Franklin (1706-1790) in his experiments with lightning (leading to his invention of the lightning rod), reasoned that the charge would travel along the metallic rod. Alessandro Volta (1745-1827) derived the concept of electrical potential from his studies of static electricity, and then discovered the principle of the battery in his experiments with dissimilar metals in common contact with moisture. Once batteries were available for contact with metals, electric currents were produced and studied. Georg Simon Ohm (1787-1854) found the direct proportion relating current and potential difference, which became a measure of the ability of various metals to conduct electricity. Extensive theoretical studies of currents were carried out by André Marie Ampère (1775-1836).

To honor these scientists, the système internationale (SI) units use their names. The unit of potential difference is the volt, and potential difference is more commonly called voltage. The unit of electrical resistance is the ohm, and the unit of current is the ampere. The relation among these functions is known as Ohm's law.

Franklin is remembered for an unlucky mistake. He postulated that there was only one type of electricity, not two as others thought, in the phenomena known in his day. He arbitrarily called one form of static electric charge positive and attributed the opposite charge to the absence of the positive. All subsequent studies continued the convention he established. Late in the nineteenth century, when advancements in both electrical and vacuum technology led to the discovery of cathode rays, streams of particles issuing from a negative electrode in an evacuated tube, Sir Joseph John Thomson (1856-1940) identified these particles as common to all metals used as cathodes and negatively charged. The historical concept of a positive current issuing from an anode is mathematically self-consistent and leads to no analytical errors, so the convention is maintained but understood to be a convenience.