Page:The New International Encyclopædia 1st ed. v. 06.djvu/871

* ELECTKICITY. 757 ELECTRICITY. phenomenon is descriliod by sayinj; that an 'elec- tric current' has passed tlirough the wire, in the direction from high potential to low. carrying the quantity of electricity c, where +e and — e were the charges in the condenser. In this state- ment there are three defuiitioiis: that of 'electric current." "direction of the current.' and the ■quantity carried by the current.' The two ef- fects of the current noted are: (1) heating ell'ect in the conductor: (2| magnetic action on a suspended magnet. These actions are observed with all electric currents; and other additional effects are obtained when the current is not steady. It should be noted, further, that, if the direction of the current in the wire is reversed, the heating ell'ect is as before, but the magnet is detlected in the opposite direction. This fur- nishes an easy method of determining tlie direc- tion of the current in a conductor. If charges can be supplied to the terminals of the long con- ducting wire as fast as they disappear — as can be done by using an electric machine or other means — the condition in the wire and around it is said to be that of a 'steady' or 'uniform' elec- tric current. In other words, to maintain a steady current in a wire, its terminals must be kept at a constant difference of potential; and the quantity of positive electricity sipplied to <ine end or of negative supplied to the other in each second of time is called the "intensity' of the current, or the "current-strength.' The quantity of electricity carried by the current is evidently the prodtict of the current-strength and the time. The difference of potential between two points on the wire is called the 'electro-motive force' (E. M. F.) between those points. It evidently re- quires energj' to maintain a uniform current, be- cause of the energy spent in thermal actions. Sources of Electric Curbexts. To maintain a steady electric current some method must be used which will produce a constant difference of potential. Four may be mentioned: (1) If an ordinary electrostatic machine is used it will be found to give a current of small intensity, the energy for the current coming from the work done in producing the charges by means of the machine. ("2) if two wires of different material, e.g. brass and iron, are joined end to end, form- ing a single circuit, there will be two junctions of brass with iron; and, if these two junctions are kept at different temperatures, there will be (in general) an electric current round the cir- cuit. The energj' for this current comes almost entirely from the source of heat at the hot junc- tion; but it may come in part from thi' fall of temperature of one of the wires. The E. .M. F. of the current is. as a rule, extremely small. (See Thebmo-Ei.ectbicity.) (3) If two ditTerent solid metal>. e.g. rods of co|)i)er and zinc, dip into a dilute solution of sulphuric aciil in water, the two metals will be found to be at a dirt'erence of potential. The same is true of any two metals dipping into any liquid — other than a fu.sed metal — which is an electric conductor. Such an arrangement is called a 'cell' or 'ele- ment.' The one mentioned has in;iny disadvan- tages, some of which may be obviated by using two liquids separated by a jiorous |)artition. (For a description of different types of cells, see VoL- T.vic Cell or Battery.) If a cell, as described above, consisting of a zinc and copp<'r rod dipping in dilute sulphuric acid is used to produce a cur- rent in a wire joining the projecting ends of the two rods, it is observed that the current in the wire is in the direction from co[)per to zinc ; that the zinc rod wastes away, and the loss in mass of the zinc is proportional to the current-strength and to the time: that bubble.-, of hydrogen gas form at the copper rod. It is known from chemi- cal e.vperiments that, when zinc is dissolved in a vessel containing dilute sulphuric acid, hydrogen gas is formed and the temperature rises, showing that in the reaction energy is given out. In the simple cell just described the zinc dissolves, hydrogen gas is formed, and the energy liberated is used in producing the electric current. A similar explanation of the supply of energy" may be given for all cells, it being noted, however, that in some cases energj' is taken from the liquid of the cell itself — apart from chemical action — as is shown bj' its tall in temperature, and in others energj' is given to the cell, as is shown bj' a rise in temperature. (4) Electric currents maj' be produced bj' dj'namos (see DvXA- mo-Electric JIachikeky), the principle of which will be examined under imlucctl Currents. nE.TiXG Effect of Currents. Since in a current through a conductor a certain quantity of electricitj' is passing from high to low poten- tial, it is losing encrgj', which in turn is gained by the conductor. Thus, if a quantitj' c has its jjotential lowered bj' V, — . the loss of energj- is c(Vi — V2) ; and this is the energv gained bv the conductor. If i is the current-strength, and ( is the time taken for a quSntitj' e to pass, e = it. Vi —, is the electromotive force and may be written E. Hence, the energy gained by the con- ductor is Elf. This goes into heating effects and is manifest bj' a rise in temperature. Illus- trations of this heating effect of a cirrent are furnished bj' the glow or incandescent lamp, bj' the arc lamp, bv electric furnaces, etc. The in- candescent lamp (see Electric Lighting) con- sists of a thin loop of compressed carbon inclosed in a glass bulb from which as much air as pos- sible has been exhausted, the two ends of the carbon filament being joined to two fine wires of platinum which enter the exhausted space from without. If. bj- means of these platinum wires an electric current is made to pass through the carbon filament, the latter will be raised to incandescence bj' the heating effect of the cur- rent; but it will not burn, because there is no oxj'gen in the bulb for it to combine with. The reason for the heating effect being so great in the filament is that it offers a great opposition to the passage of the current, and so a great amount of energj' is necessary to force the current through. The arc lamp consists of two carbon rods so kept bj' automatic devices that there is a small gap between them. When the lamp is started the rods are in contact, and a current is forced through from one rod to the other: there is a great rise in temperature at the point of con- tact, owing to the great resistance to the passage of the curient — this ntakes the air a good con- ductor; then the poles are drawn a slight dis- tance apart automatically; and owing to the resistance to the current passing from the carbon to the air. the carbon tips are heated white-hot and emit light. Magnetic Action of a Cfrrent. If a long, straight wire earrving a current is held over and parallel to a pivoted magnetic needle, the needle will be dcllected to one side or the other, depend-