The C.G.S. System

The table, p. 18, shows the method of derivation of such absolute units on the C.G.S. system as we shall have occasion to make use of in this book. The first column contains the denominations of the quantities measured; the second contains the verbal expression of the physical law on which the derivation is based, while the third gives the expression of the law as a variation equation; the fourth and fifth columns give the definition of the C.G.S. unit obtained and the name assigned to it respectively, while the last gives the dimensional equation. This will be explained later (p. 24).

The equations given in the third column are reduced to ordinary equalities by the adoption of the unit defined in the next column, or of another unit belonging to an absolute system based on the same principles.

Some physical laws express relations between quantities whose units have already been provided for on the absolute system, and hence we cannot reduce the variation equations to ordinary equalities. This is the case with the formula for the gaseous laws already mentioned (p. 15).

A complete system of units has thus been formed on the C.G.S. absolute system, many of which are now in practical use. Some of the electrical units are, however, proved to be not of a suitable magnitude for the electrical measurements most frequently occurring. For this reason practical units have been adopted which are not identical with the C.G.S. units given in the table (p. 20), but are immediately derived from them by multiplication by some power of 10. The names of the units in use, and the factors of derivation from the corresponding C.G.S. units are given in the following table:

Table of practical units for electrical measurement related to the C.G.S. Electro-magnetic system.
Quantity	Unit	Equivalent in C.G.S. Units
Electric current	Ampere	10-1
Electromotive force	Volt	108
Resistance	Ohm	109
Capacity	Farad	10-9
Rate of working	Watt	107
Quantity of Electricity	Coulomb	10-1

To shorten the notation when a very small fraction or a very large multiple of a unit occurs, the prefixes micro- and mega- have been introduced to represent respectively division and multiplication by 10⁶. Thus:

A mega-dyne = 10⁶ dynes
A micro-farad = 10^-6 farad.