International System of Electrical and Magnetic Units

The International System of Electrical and Magnetic Units is an obsolete system of units used solely for measuring electrical and magnetic quantities. It was introduced by the Fourth International Conference of Electricians (Chicago, 1893) and modified in 1908. It was rendered obsolete by the inclusion of electromagnetic units in the International System of Units (SI) in 1948.

Earlier systems

The link between electromagnetic units and the more familiar units of length, mass and time was first demonstrated by Gauss in 1833 with his measurement of the Earth's magnetic field. A complete system of metric electrical and magnetic units was proposed by Weber in 1851.

1893 system

The International System was introduced in 1893 because of the practical difficulties in measuring electrical units in the cgs system. The 1893 system had three base units: the international ampere, the international ohm and the international volt.

Unit	1893 ("international") definition	cgs ("absolute") equivalent	Notes
Ampere	the unvarying electric current that will deposit 0.001 118 00 grams of silver per second from a solution of silver nitrate in water	the current produced in a conductor with a 1 ohm resistance when there is a potential difference of 1 volt between its ends
Ohm	the electric resistance of a column of mercury of constant cross-section at the temperature of melting ice, 106.3 centimetres long and with a mass of 14.4521 grams		equal to 109 cgs units of electric resistance
Volt	such that the electromotive force of a Clark cell at a temperature of 15 °C is exactly 1.434 international volts	the electromotive force produced in an electric circuit which cuts 108 magnetic lines of force per second	equal to 108 cgs units of electromotive force

Overdefinition and the 1908 modification

The 1893 system of units was overdefined, as can be seen from an examination of Ohm's law:

V = IR

By Ohm's law, knowing any two of the physical quantities V, I or R (potential difference, current or resistance) will define the third, and yet the 1893 system defines the units for all three quantities. With improvements in measurement techniques, it was soon realized that 1 V_int ≠ 1 A_int × 1 Ω_int.

The solution came at an international conference in London in 1908. The essential point was to reduce the number of base units from three to two by redefining the international volt as a derived unit. There were several other modifications of less practical importance:

• the international ampere and the international ohm were formally defined in terms of the corresponding cgs electromagnetic units, with the 1893 definitions retained as preferred realizations;
• the preferred realization of the international volt was in terms of the electromotive force of a Weston cell at 20 C (1.0184 V_int), as this type of cell has a lower temperature coefficient than the Clark cell.
• several other derived units for use in electrical and magnetic measurements were formally defined.

External links

• Sizes.com