Metric-c.g.s., also CGS system
(Metric) A version of the general system that has its derived constants relating coherently to the centimetre, the gram, and the second, in contrast with the metre, the kilogram, and the second of the m.k.s. system and its contemporary form, the SI system. In the c.g.s. system, for example, the unit of force is the dyne, which gives an acceleration of 1 cm·s-2 to a body of 1 g, so is a force just one hundred-thousandth of the newton of the SI, which gives an acceleration of 1 m·s-2 to a body of 1 kg. (The crucial factor favouring c.g.s. appears to have been that the gram was the mass of a cubic centimetre of water.
[Nature Vol. 39, 18-19 (1873)]) Various formulations for the associated electrical units were introduced: initially the electromagnetic (e.m.u. system) and the electrostatic (e.s.u. system), which based all their units on mechanical equivalence, then, deriving from the former but with some base definitions of electric units, the practical units and international units. The gaussian system was a hybrid of the former pair. Agreed at the first International Electrical Conference in 1881,
[Nature Vol. 24, 512 (1881)] the c.g.s. system is now obsolete. Base units were, for
• length: cm = centimetre;
• mass: g = gram;
• time: s = second;
• luminous intensity: cd = candela, originally new candle.
The derived coherent mechanical units common to all forms of the c.g.s. system were, for
• acceleration of free fall: gal = cm·s-2 (= 10-2 m·s-2);
• acoustics - specific impedance: rayl = dyn·s·cm-3 (= 10-1Pa·s·m-1 = 10 N·s·m-3);
• acoustics - mechanical impedance: mechanical ohm = dyn·s·cm-1 (= 10-3 N·s·m-1);
• acoustics - mechanical mobility: mohm = cm·dyn-1·s-1 (= 103 m·N-1·s-1);
• dynamic viscosity: P = poise = dyn·s·cm-2 = g·cm-1·s-1 (= 10-1Pa·s);
• energy, work, quantity of heat: erg = dyn·cm = cm2·g·s-2 (= 10-7 J);
• force: dyn = dyne = cm·g·s-2 (= 10-5N);
• illuminance: ph = phot = cd·sr·cm-2 (= 104lx);
• kinematic viscosity: St = stokes = cm2·s-1 (= 10-4 m2·s-1);
• luminance: L = lambert = cd·cm-2 (= 104lx);
• luminous emission: sb = stilb = cd·cm-2 (= 104 cd·m-2);
• mechanical impedance: mechanical ohm = dyn·cm-1·s (= 10-3 N·s·m-1);
• pressure: barad = dyn·cm-2 = cm-1·g·s2 (= 10-1Pa);
• quantity of light: lumerg = cd·sr·s (= 1 lm·s);
• specific acoustic impedance: rayl = dyn·s·cm-3 (= 10 N·s·m-3).
For electromagnetic units, see electromagnetic unit, electrostatic unit, and Gaussian unit; see also practical unit.
Non-coherent units used included bar, calorie, darcy, langley, leo, pyron, and rad.
