The System.— The centimeter-gram-second system of units is so called because it is based upon the centi meter as the unit of length, the gram as the unit of mass and the second as the unit of time. The name of the system is commonly abbie viated to "C.G.S.' In the C.G.S. system, the unit of force is the "dyne"; a dyne being defined as the force which, when allowed to act for one second upon a body having a mass of one gram, will communicate to that body a velocity of one centimeter per second. The unit of work in the C.G.S. system is the "erg," this being defined as the quantity of work that is done when a force of one dyne is overcome through a distance of one centimeter. The unit of energy is also the erg, because energy is measured by the quantity of work that it can perform. The dyne is a very small unit in comparison with the forces that we commonly have to deal with in the physical world, it being only about 2 per cent greater than the attraction that the earth exerts upon a milligram of matter. To avoid the use of inconveniently large numbers in expressing forces, a unit called the "megadyne is, therefore, used to some considerable extent, a "megadyne being defined as equal to 1,000,000 dynes. This convention of prefixing "mega-" to indicate a unit of 1,000,000 times as great as the unit whose name follows the prefix is quite common. A quantity of work equal to 1,000,000 ergs, for example, is called a "megaerg," or more com monly a megerg," or a "megalerg." The prefix "kilo-') is similarly used to signify a unit that is 1,000 times as great as the unit to the name of which it is prefixed; this being famili arly illustrated in the "kilogram" and the "kilometer." The prefix "micro?) is also em ployed to a considerable extent, to signify that the unit to which it relates is one one-millionth as great as the unit to the name of which it is prefixed. A "microfarad," for example, is a unit having a magnitude equal to the millionth part of a farad.
The most arbitrary unit in the C.G.S. system is the "absolute atmosphere," which is defined as the pressure of a megadyne per square cen timeter. This unit, however, has the advantage of being perfectly definite. Moreover, it is not greatly different from the ordinary pressure of the atmosphere. At sea-level in the latitude of Paris, for example, a column of mercury 76 centimeters high, and at a temperature of C., exerts a pressure of 1.0136 megadynes per square centimeter; that is, a pressure of 1.0136 "absolute atmospheres." All of the electrical and magnetic units that are now in common use are based upon the centimeter-gram-second system; and they illuA trate, admirably, the convenience of that system.
Electrical and Magnetic unit magnetic pole, in the C.G.S. system, is a mag netic pole of such strength that it will repel an equal pole, at a distance of one centimeter, with a force of one dyne. The intensity of a mag netic field, in this system, is numerically equal to the number of dynes of force that the field will exert upon a unit pole that is placed in it. The of a magnet is the product of the distance between its poles (expressed in centimeters) by the strength of one of these poles; and the intensity of magnetization of a magnet is numerically equal to the moment of the magnet, divided by the volume of the mag net in cubic centimeters.
In electrical measurements, two distinct sets of units are employed, these being known, re spectively, as the "electrostatic units" and the "electromagnetic units," because one is com monly employed in calculations concerning static electricity, while the other is employed in work that relates to dynamic (or elec tricity. Both sets are based upon the C.G.S.
system, and it is possible that in the not very distant future a single set of units will be em ployed for electrical measurements of all kinds. At present, however, this is not feasible, on account of our temporary ignorance with re spect to certain points which must be cleared up before the ideal single set of electrical units can be realized. Consult Lodge, 'Modern Views of ; Maxwell, 'Electricity and Mag netism.) Electrostatic The absolute (or C.G.S.) units for the measurement of static electricity are derived ,as follows: The C.G.S. unit of "quantity of electricity" is that quantity which would repel an equal quantity, situated at a distance of one centimeter, with a force of one dyne. The C.G.S. unit of electromotive force (or difference of potential) is that dif ference of potential through which a unit quan tity of electricity must be raised, in order that the work done shall be one erg. The capacity of a conductor, in C.G.S. measure, is the quo tient obtained by dividing the quantity of elec tricity upon the conductor by the potential which this quantity of electricity produces in the conductor. (The capacity of an isolated sphere is numericallyequal to the radius of the sphere, as expressed in centimeters). The unit of current is the current which conveys a unit quantity of electricity in one second of time.
Electromagnetic The units that are employed in the measurement of dynamic (or current) electricity are derived as follows: The unit of current is the current which, when flowing along a circular arc one centimeter in length and one centimeter in radius, produces, at the centre of the arc, a magnetic field of unit intensity. The unit of "quantity of elec tricity" is the quantity of electricity that a unit current conveys in one second. The unit of electromotive force (or of difference of potential) is a difference of potential of such magnitude that to cause the flow of a unit quan tity of electricity against it requires the ex penditure of one erg of work. The unit of re sistance is the resistance through which a unit current would be produced, by one unit of elec tromotive force. The unit of capacity is the capacity of a conductor whose potential is in creased by unity, by the addition to the con ductor of one unit of electricity.' The absolute electrical units, in the electrostatic and electro magnetic systems, have not (in general) received definite names; it being sufficient, in calcu lations in which these units are employed, to speak of a certain number of "C.G.S. units of electricity," or "C.G.S. units of resistance," etc. For the practical measurement of dynamic elec tricity in the laboratory and the power-house, the absolute (or C.G.S.) units are not of con venient size; and for them it is, therefore, cus tomary to substitute certain more convenient multiples and submultiples of the electromag netic units, as they are defined above. The modified units are known as the "ohm," "ampere," "volt," etc., and are considered in the following paragraph.