siemens

Two electrical units, the second of which is obsolete:

1

In SI and in the meter-kilogram-second system, the unit of electrical conductance, admittance, and susceptance. Symbol, S. A conductor has a conductance of 1 siemens if an electrical potential difference of 1 volt produces a 1-ampere current in it. The conductance of a conductor in siemens is the reciprocal of its resistance in ohms; the siemens was formerly known as the mho or reciprocal ohm. In equations, conductance is represented by G.

“Siemens” is the singular and plural; “1 siemen” is wrong.

The dimensions of the siemens are $The fraction amperes over volts$ ,

or, in terms of base units only, $A fraction whose numerator is seconds cubed times amperes squared and whose demominator is meters squared times kilograms$

The 14th CGPM added the siemens to SI in 1971. The siemens is named for the German engineer, Werner von Siemens.

2

In the 19th century, the Siemens’ unit or Siemens’ mercury unit was a unit of electrical resistance introduced in 1860 by Werner von Siemens himself.¹ One Siemens’ unit is approximately 0.9534 ohm. The standard was defined as the resistance of a column of pure mercury 1 meter long with a cross sectional area of 1 square millimeter, at a temperature of 0°C. For everyday purposes, the standard was realized as a German silver wire 3.8 meters long and 0.9 millimeters in diameter.²

Siemens manufactured reference wire coils calibrated in this unit.

The advantages of the Siemens' unit were that its magnitude was convenient for telegraph engineers and that a high precision standard was fairly easily constructed in the laboratory. The disadvantage was that its definition was entirely unrelated to definitions of units of voltage or current, which complicated calculation.

The Siemens' unit was one of two units of resistance that competed for acceptance (the other being the B.A. unit of resistance) until the definition of the ohm at the First International Electrical Congress (Paris, 1881), after which the siemens’ unit withered.

1. Werner Siemens.
Proposal for a new reproducible standard measure of resistance to galvanic currents.
Philosophical Magazine, volume 23, pages 171–179 (March 1862)
Translated from Annalen der Physik, Jan 1860.

2. Ganot's Physics. See below.

resources

Werner von Siemens
Inventor and Entrepreneur: Recollections of Werner von Siemens.
London: Lund Humphries, 1966.

Examples

It is of great scientific and practical importance to have a unit or standard of comparison of resistance, and numerous have been proposed. Jacobi proposed the resistance of a metre of a special copper wire a millimetre in diameter. Copper is, however, ill adapted for the purpose, as it is difficult to obtain pure. Matthiessen proposed an alloy of gold and silver, containing two parts gold and one of silver; its conducting power is very little affected by impurities in the metals, by annealing, or by moderate changes in temperature.

Siemens' unit is a metre of pure mercury, having a section of a square millimetre. Its actual material reproduction for ordinary use is a German silver wire 3.8 metres in length and 0.9 mm. in diameter. It is 0.9534 of the ohm.

E. Atkinson.
Elementary Treatise on Physics Experimental and Applied for the use of colleges and schools. Translated and edited from Ganot's Éléments de Physique. Twelfth edition.
London: Longmans, Green, and Co., 1886.
Page 923.