Et.r.croie.xt. Electric charges may be produced by various render the processes of charging by contact or by induction continuous and automatic. The com monest. forms are the frictional machine (see ELEcritiC.1.1.. :At:WHINES). the civet rOpliOrtis the induction machine.
Instruments have been devised to measure dif ference of electric potential both absolutely and relatively. They a re called elect rometers, and are described under that title elsewhere. See E Lhcrttoxi h'rEns.
TunEs. eonvenient mode of de scribing eha•trical phenomena is due to Faraday. who pictured the electric field round charged bodies as filled with tubes whose side, were made by lines of force. These tubes join positive with negative charges, and are chosen of such tt cross section that where they Meet t charged surface the open end of each tube Meioses a unit eharge. The tube will have in general a Varying er11,1 cq. depending, on the paths of the lines of force, and where the electric force is the great est the tubes are most numerous.
ELE("rnoKINETICS, FUNILVAIENT.kl. PlIENoAIEN.t. Tf a charged electrical condenser is discharged by joining its two conductors or 'plates' by a /on!, conducting wire, it is observed that there is no oscillation. but that the charges disappear and there are certain phenonama in and near the wire. The temperature of the win. is and a magnet sospended. free to turn, near the wire and parallel to it. will be (101(.0(•1 so as to tend to stand at right angles to it. In special eases, other phenomena may be observed. The whole phenomenon is described by saying that an 'cloy tric current' has passed through the wire. in the direction from high potential to low, carrying the quantity of electricity r, where and —e were the charges in the condenser. In this state•
mcm there are three definitions: that of 'electric current; 'direction of the •urrent,' and the 'quantity carried by the current.' The Iwo ef of the current noted are: I11 heating effect in the conductor: t2t action ou a suspended magnet. These actions are observed with all electric currents; and oilier additional effect, 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 effect is as before, hut the magnet is deflected in the opposite direction. This fur nishes an cagy method of determining the direc tion of the current in a conductor. If charges can be supplied to the terminals of the long con dinging wire as fast as they disappear—as can be done by using an electric machine or other means—the condition in the wire aml around it is said to he that of a 'steady' or 'uniform' elee trio current. In other word-. to maintain a steady current in a wire, its terminals must be kept at a con,tant difference of potential: and the quantity of positive electricity supplied to one 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 product of the current-strength and the time. The difference of potential between two points on the wire is called the 'clectro-motive force' ( E. AI. F.) between those point,. it evidently re quire- energy to maintain a uniform current, be cams(' of the energy spent in thermal actions.