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# 16: CGS Electricity and Magnetism

• • Contributed by Jeremy Tatum
• Emeritus Professor (Physics & Astronomy) at University of Victoria

An older system of units, still used by some authors, was the CGS (centimeter-gram-second) system. In this system, a dyne is the force that will impart an acceleration of 1 cm s-2to a mass of 1 gram. An erg is the work done when a force of one dyne moves its point of application through 1 cm in the line of action of the force. It will not take the reader a moment to see that a newton is equal to 105 dynes, and a joule is 107 ergs. As far as mechanical units are concerned, neither one system has any particular advantage over the other.

• 16.1: Introduction
We are accustomed to using MKS (metre-kilogram-second) units including meters, kilograms, and seconds. For electricity and magnetism, we extended the MKS system by adding an additional unit, the ampère, to form the MKSA system. This in turn is a subset of SI (le Système International des Unités), which also includes the kelvin, the candela and the mole.
• 16.2: The CGS Electrostatic System
• 16.3: The CGS Electromagnetic System
One CGS emu of magnetic pole strength is that pole which, if placed 1 cm from a similar pole in vacuo, will repel it with a force of 1 dyne. The system is based on the proposition that there exists a "pole" at each end of a magnet, and that point poles repel each other according to an inverse square law.
• 16.4: The Gaussian Mixed System
A problem arises if we are dealing with a situation in which there are both “electrostatic” and “electromagnetic” quantities. The “mixed system”, which is used very frequently, in CGS literature, uses esu for quantities that are held to be “electrostatic” and emu for quantities that are held to be “electromagnetic”, and it seems to be up to each author to decide which quantities are to be regarded as “electrostatic” and which are “electromagnetic.
• 16.5: Conversion Factors
• 16.6: Dimensions