meter-kilogram-second

(Meter-Kilogram-Second system) A metric system of measurement that uses the meter, kilogram, gram and second for length, mass and time. The units of force and energy are the "newton" and "joule." See SI, newton and joule. Contrast with CGS system.

Download Computer Desktop Encyclopedia to your iPhone/iTouch

(Metric) The version of the metric system, notably now in the form of the SI system, that has its derived constants relating coherently to the metre, the kilogram, and the second, in contrast with the centimetre, gram, and second of the long-used c.g.s system, for instance. In the m.k.s. system the unit of force, for example, is the newton, which gives an acceleration of 1 m·s^-2 to a body of 1 kg, rather than the dyne of the c.g.s. system, which gives an acceleration of 1 cm·s^-2 to a body of 1 g, hence of only 10^-5 m·s^-2 to a body of 1 kg.

The m.k.s. basis for coherency began by accident, in the form of the practical units of electromagnetics in the 19th century, derived by relatively arbitrary scaling of the electromagnetic units of the c.g.s. system. Their coincident fit to m.k.s. was realized and publicized in 1901 by Giorgi, who stimulated its adoption generally to the mechanical and other fields. What was originally known as the Giorgi system was accepted by the International Electrotechnical Commission in 1933 as appropriate for electro-technicians without affecting the use of the classical c.g.s. system by physicists.
[Kennelly A. E. Elect. Engng Vol. 53, 402-5 (1934)] In 1946 the CIPM decided that, effective from the beginning of 1948, an m.k.s. system would displace c.g.s. as the primary system for science, etc. In 1960, the adoption of the SI system brought a new title; ‘m.k.s.’ survived as the formula for coherency, but the label became historic.

When the label first came into use, Kg stood for kilogram. Now, under the SI, the symbol for kilo- is k. So m.k.s. is correct today, but m.K.s. was correct yesterday.

The system adopted in 1948 incorporated the ampere as a base unit; it could, for instance, have been the volt instead, or, as with the e.m.u. system, could have related all electrical units to their mechanical equivalents, obviating the definition of any base electric unit. In consequence of this decision, the SI is called an m.k.s.A. system (q.v. for further notes on the use of lower- versus upper-case letters in the system title).

Most of the older derived units of m.k.s. survive in the SI (the mho as the siemens), exceptions among coherent units being, for:

• acoustics - specific impedance: rayl = N·s·m^-3 = Pa·s·m^-1;
• acoustics - mechanical impedance: mechanical ohm = N·s·m^-1;
• acoustics - mechanical mobility: mohm = m·N^-1·s^-1;
• dynamic viscosity: Pl = poiseuille = N·s·m^-2 = Pa·s;
• luminance: nit = cd·m^-2;
• luminous energy: talbot = s·cd·sr.

Non-coherent units of the old m.k.s. system included apostilb, bar, darcy, langley, mayer, poncelet, and rad.

meter-kilogram-second system