ITEATING EFFECT OF CFURENTS. Since in a current through a conductor a certain quantity of electricity is passing from high to low poten tial, it is losing energy. which in turn is gained by the conductor. Thus, if a quantity e has its potential lowered by V,— V. the loss of energy is V2) : and this is the energy gained by the Conductor. If i is the current-strenrth, and is the time taken for a quftntity c to pass, c = it. V,—V, i- the electro-motive force and may he written E. 'Hence, the ellergy gained by the con ductor is Eit. This goes into heating effect and is manifest by a rise in temperature. Illus trations of this heating effect of a current are furnished by the glow- or incandescent lamp. by the arc lamp. by electric furnaces, etc. The in candescent lamp (see ELEcTRic LIMITING) con 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 whidt enter the exhausted space trout without. If. by means of these platinum wire an electric- current is made to pas, through the carbon filament, the latter will be raised to ineandescenee by the beating effect of the cur but it will not burn, became there is no oxygen 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 energy is necessary to force the current through.
The are lamp consists of two carbon rod; Si) kept by antomatie devices that there is a -mall gap between them. the lamp i- started the rods are in contact. and a current is fo•eed through front one rod to the other: there is a great rise in temperature at the point of cam tact, owing to the great resistance to the passage of the •urrent—this makes the air a good con ductor: then the poles are drawn a slight dis tance apart automatically: and owing to the resistance to the en•rent passing from the ca•hmt to the air. the carbon tips are heated white-hot and emit light.
nyiNEric ACTION OF A CURRENT. If a long. straight wire earryhm a current is held over and parallel to a pivoted magnetic needle, the needle will be deflected to one side or the other, depend.
ung upon the direction of the current. The sim plest statement of the la W of the magnetic action of a current is that a wire carrying a current is surrounded by rings of magnetic lines of force, the connection being such that, if a right-handed screw be imagined placed so is to coincide with the conductor. and if it is turned in the manner
indicated by the line of magnetic force, its motion of translation will be in the direction of the current.
If the conductor is bent in the form of a loop, lines of magnetic force will enter the loop on one side and return outside the loop, each line forming a elof-wd curve. Thus the face of the loop which the lines enter is like the south pole of a magnet, and the opposite face is like the north pole. In other words, the magnetic action of the loop of wire carrying a eurrent is like a thin sheet of iron filling the area of the loop, and so magnetized that all the north poles are on one face and the south poles on the other.
Shnilarly, if a wire is wound in the form of a helix or spiral spring, the lines of magnetic force will enter at one end of the helix, pass along its length, and. emerging• from the opening at the farther end, return outside, forming closed curves. A helix like this when it carries a cur rent has, therefore, the properties a liar net. If a small magnet is placed inside this helix it will turn and place itself along the line of magnetic force; similarly, if a rod of soft iron or other magnetic substance is wound with an in wire helix carrying a current, it will be magnetized. Such a magnet is called an 'eke tro-magnet,' and in various forms they play an essential part in call-bells. telegraph, and tel instruments, dynamos, motors. etc. The bar of iron is generally so bent that its two ends face each other: sometimes it forms a 'horseshoe.' One of its ends is then a north magnetic pole, and the other a south pole whenever the electric current is passed through the enveloping helix of wire. Since conductors carrying currents have the magnetic properties of magnets, they have forces of attraction and repulsion on each other and on magnets. It may lie shown that all these various eases of force may lie embraced in the statement that "motions take place in such a manner that the circuits of emiduetors inclose the greatest possible monher of lines of magnetic force emerging from the north faces of the eir (mit s, or the least number of lines emerging from the south faces." The action of electric motors is based on this principle. See DYNAMO-ELECTRIC ntCIIINERY.
Many instruments for the detection or meas urement of electric currents are based upon their magnetic action. if a magnet is pivoted so as to he free to turn around a vertical axis, and if it is placed in the plane of a loop of wire, it will he deflected if a current is passed through the loop, thus making a 'galvanoseope.' (See