ELECTRICAL MACHINE. 748 ELECTRICAL UNITS. and other a]ipli;iiiccs. In the best of these machines spuiUs over a foot in length are ob- taini'il. In electrical machines, as in the case of all ajiparatus where electricity at high potential is used, the greatest care must be observed in insulating the ditVcrent parts and conductors. Particularly, all glass parts on which the mois- ture of the atmosphere is likely to settle must be covered with shellac, varnish, or parallin. Electrical machines are available for many ex- periments in static electricity, but save for ig- niting gas-jets for use in electrotherapeutics they have few practical applications in ordinary life. They will be found described in a simple manner in S. P. Thompson, Elei>ientnry Lessons in Electricity and Muf/netism (Xew York, lilOl). An interesting historical discussion is given in Gerland and Traumiiller, Geschiclite der Physik- alischen Experitnrnticrl:unst (Leipzig, 1809). The advanced student may consult with profit the various volumes of the Annalen der Plii/sik, and such treatises as MiillerPouillet-Pfaundler, LchrhiicU der Phi/sik, vol. iii. (Brunswick, 1888- 90) : Wiillner, Lehrbuch der Kxiierimcntiil Plii/sik (Leipzig, 18SG) ; and Gustav Wiedcinan, Die Lehre von der Elcctricitiit (Brunswick, 1882-85). See articles Eiectuicity: Ei.ECTROPiioRr.s. ELECTRICAL UNITS. In order to express by mimbcrs tlie amounts of the different electric and magnetic quantities, various units or stand- ards of comparison are necessary. They are all based upon the definition of a unit electrical charge and of a unit magnetic pole. TiiK C. G. S. Ei.ECTno.sTATic system starts with the following definition: A unit electric charge is such a one that if two particles of matter, placed in a vacuum at a distance of one centimeter apart, are each charged with a unit charge, the force of action of one on the other will l)e one dyne. Based on this definition are the following tmits: Two points are at a unit difference of potential, if it requires one erg to carry a unit charge from one point to the other. The capacity of a condenser is unify, if, when charged to a unit difference of potential, there is a unit charge on each plate of the condenser. A imit current is such that if it flows for one seeonil. an electric charge cipial to unity is carried. On tliis >mit current all the magnetic quantities could be based. The C. <t. S. ELECTRIC-1I.A.GXETIC system starts with the following definition: A unit magnetic pole is such that if two such poles are at a dis- tance apart of one centimeter in a vacuum, the force of action of one on the other is one dyne. On this definition is then based that of a unit electric current, viz., it is such a current that, if it is flowing through a linear conductor bent in the form of a circle of radius r cm., the force on a unit magnetic pole placed at the centre of the circle is 2 jt /r, where as usual jr = 3.14159. A unit quantity is then the charge carried by a unit current in one second, and to this can be referred units of potential and capacity: a unit electro- motive force between two points is such that it requires one erg to carry a unit quantity between them, or if a imit current is flowing between them an amount of encrgj' equal to one erg is set free in one seccmd: a linear conductor of unit re- sistanceis such that if a unit electro-motive force is applied at its ends a unit current will be pro- duced : a unit of induction is the inducticm in a circuit when the electro-motive force induced in this circuit is unity while the inducing cur- rent varies at the rate of a unit current per second. Experiments have established the connectiou between the units for the same quantity in the two systems. Knowing this ratio for any ([uan- tity, e.g. electric current, it may be at once de- duced for the others. It is thus established that Electrostatic unit Kleetro-magnetic unit of quantity or current = of quantity or current-:- v of potent ial or E.M.F.= of potential or E.M.F.X" of capacity = of capacity -j- r' where r = 3 X 10'° or 30,000,000,000 very ap- proximately. This is the same number as that ex- pressing the velocity of light as it should be ac- cording to the electromagnetic theory of light, whicii identities the luminiferous ether with the medium serving for the propagation of disturb- ances due to electric oscillations, ^ee Liuirr; Electricity. These C. G. S. electro-magnetic and electrostatic unitsare, in nearlyevery case, of inconvenient mag- nitude for practical purposes; and. further, their definitions are not such as lead easily and direct- ly to methods of measurement. Consequently, certain so-called 'practical units' have been adopt- ed which are of convenient magnitude and bear simple relations to the absolute units, so far as possible. The following are these jiractical units, as legally defined: The unit of resistance shall be what is known as the international ohm. which is substantially equal to 1,000.000.000 units of resistance of the centimeter-gram-second system of electro-magnetic units, and is re])resented l)y the resistance offered to an unvarying electric current by a column of mercury at the tempera- ture of melting ice, 14.4521 grams in mass, of a constant cross-settional area, ;>nd of the length of 106.3 centimeters. (This would recpiire a cross-section of 1.00003 square milliineters. ac- cording to the accepted value for the density of mercury.) One million ohms — i.e. 10° ohms, is called a 'megohm.' The unit of ctirrent shall be what is known as the international am|)ere. which is 0.1 of the unit of current of the centimeter - gram - second system of electro-magnetic units, and is the prac- tical equivalent of the nnvari'ing current, which, when passed through a solution of nitrate of silver in water, in accordance with standard specifications (see Ampere), deposits silver at the rate of 0.001118 of a gram per second. One thousandth of an ampere is called a milliamperc. The unit of electro-motive force sli;ill be what is known as the international volt, which is the electro-motive force that, steadily ap))lied to a conductor whose resistance is one international ohm. will produce a current of an international ampere, and is practically equivalent to {^"JJ of the electro-motive force between the poles or electrodes of the voltaic cell known as Clark's cell, at a temperature of 15° C. and prepared in the manner described in the standard specifica- tions. 8ee Volt. The unit of quantity shall be what is known as the intern;itional coulomb, which is the quantity of electricity t ransfcrred by a current of one inter- national ampere in one seconcl. See CofLOMn. The >init of capacity shall be what is known as the international farad (see F.vrad). which is the capacity of a condenser charged to a po- tential of one international volt by one inter- national coulomb of electricity. One-millionth of a farad is called a microfarad.
