ELECTRICITY. 758 ELECTRICITY. ing upon the direction of the current. The sim- plest statement of the law 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 as to coincide with the eondxictor, 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. Jf 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 closed 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 current is like a thin sheet of iron lllling the area of the loop, and so magnetized that all the north poles are on one face and the south poles on the other. Similarly, 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 heli.x like this when it carries a cur- rent has, therefore, the ])ropertips of a bar mag- net. Tf 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 insulated wire helix carrying a current, it will be magnetized. Such a magnet is called an 'elec- tro-magnet,' and in various forms they plaj' an essential part in call-liells, telegraph, and tel- ephone instruments, dynamos, motors, etc. The bar of iron is generally so lient 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 be shown that all these various cases of force may be embraced in the statement that "motions take place in such a jnanncr that the circuits of conductor^; inclose the greatest possible inimbcr of lines of magnetic force emerging from the north faces of the cir- cuits, or the least number of lines emerging from the south faces." The action of electric motors is based on this principle. See Dynamo-Electbio Maciunehy. Miiiiy instruments for the detection or meas- urement of electric currents are based upon their magnetic action. If a magnet is pivoted so as to be free to turn around a vertical axis, and if it is placed in the plane of a loop of wire, it will be deflected if a current is passed through the loop, thus making a "galvanoscopc' (See Galvanometer.) The delicacy of the instrument may be increased in m:uiy ways: making the magnet light and short, winding the wire in sev- eral turns instead of a single loop, siispending the magnet by a fine fibre with little if any torsional resistance, making the magnet 'astatic.' An 'astatic' magnet is really a combination of two (or more) nuiLTiets rigidly fastened to a stifT rod or 'staff.' and so placed (hat the directive magnetic action of the earth is destroyed as far as possible. This is done by placing the magnets in oi>l>osile directions, but in the same plane, and making them as nearly as possible of the same strength. In suspending this compound system in the galvanoscopc it is so arranged that one magnet (or set of similar m.agnets) comes inside the coil of wire, while the other magnet (or set of magnets), turned in the opposite direction, lies outside the coil. In another tyjie of instrument a flat coil of wire is suspended by a wire which has torsional rigidity between the two poles of a horseshoe magnet, so that the line joining the poles is par- allel to the faces of the coil. When a current is passed through the coil it will turn so as to tend to place its faces perpendicular to the lino joining the poles, thus including more lines of force. The rigidity of the suspension prevents the coil from turning far ; and the turning moment due to tlie action of the magnet <jn the current is balanced by the moment of restitution of the twisted wire. This is the ])rinciple of the siphon recorder, of the D'Arsonval galvonometcr, etc. Still another form of instrument difl'ers from the one just described in having the niapietic field due to the permanent magnet replaced by one due to an electric current through ])arallel coils of wire, one on each side of the suspended coil, with their faces at right ;uigles to those of the latter. This instrument is called an 'elec- trodynamometer.' It should be noted that Rowland has shown by direct experiment that an electrostatic charge carried at a high speed has the same efTect on a magnet as a current: the direction of the equiva- lent current depending upon whether a positive or a negative charge is carried. ELECTR0LY.S1S. Jletal wires are not the only conductors for electric currents, although they are (be ones most conunonly seen. Many lirpiids are conductors, and all gases may be nnule (o conduct. To pass a current through a liquid or a gas. wires must be used so as to conduct the current in at one point and out at another. The wire by which the current enters is called the 'anode': that by which it leaves, the 'cathode.' Fused metals conduct like solid metals; but in (he case of o(her liquid conduc(ors (and also some solid ones) there are chemical actions at the anode and cathode: gases are evolved or solid ma((er is deposited on (he nic(al wires or pla(es. Such conductors are called 'electrolytes,' ami the process of conduction through them is called 'electrolysis.' If all electrolytes are examined it is found that they arc the solutions of certain salts or acids in some liquid — such as water: and that there is evidence of the dissociation of (he salt or acid by (he ac(ion of the liquid. Fara- day found by studying the rcladve quantities of matter separated out from the liipiid at (he cathode and anode, when an electric current was passed through an electrolyte, (wo laws which bear his name: (1) Tlie mass of matter separated at either cathode or anode in any one elec(rolyte varies directly as the quantity of electricily car- ried by the current, i.e. as the product of (he current-strength and the time. (2) If (he same current is passed through several electrolytes in series, the masses of (he ma((er separa(ed oii( at (he different anodes or cathodes vary directly as the 'chemical equivalents' of (he ma(ter. The chemical equivalent of any element is its 'atomic weight' (see .Atomic- Wkioiit.s) ili- vided by its 'valence.' For example, the chemical
