Page:The American Cyclopædia (1879) Volume VII.djvu/608

 696 GALVANISM the other. A binding screw connects with the metal cylinder, and when this and the other binding screw are' connected with the elec- trodes of a battery a galvanic current will pass through the wire which is wound upon the non-conductor, and also through the metal cyl- inder, so that it will be easy to interpose in the circuit any desired length of wire having any desired area of cross section. Establishing a certain length of a certain sized wire as a unit of measure, a comparison may be made between the resistances of various media. To measure the resistance of any conductor, the rheostat and sine galvanometer may be used in tbe following manner : In fig. 22, let m be a conductor whose resistance is to be measured or compared. One end of it is dipped hi a cup of mercury, 5, which is also connected with one pole of a battery, T. The other end of m dips into a second cup of mercury, a, which is connected with one of the binding screws of the rheostat. A wire attached to the other binding screw is connect- ed with one end of the wire which passes around the vertical circle of the galvanometer, the other end of which connects with the other pole of the battery. The rheostat wire is all wound on the metal cylinder, and the circuit being closed, the deflection of the galvanometer FIG. 22. may be noted. Then the conductor m is re- moved from the circuit, and the two wires at a and 5 are joined. Enough of the rheostat wire is now wound on the non-conducting cyl- inder to cause the same deflection in the needle as before. That portion of the rheostat wire through which the current passes will have the same resistance as that of the conductor m, whose amount is therefore found by compari- son. The results obtained from numerous ex- periments upon the conductivities of various metals show that silver, gold, and copper are the three best conductors, and that impurities great- ly increase resistance, as will also an increase of temperature. It has been shown by Forbes that metals have proportional conductivities for heat and for electricity, and that impurities also proportionately increase the resistance for each. The following table gives E. Becquerel's determinations of specific electrical resistances at 15 0., regarding that of silver at as 100 : Silver 107 Copper 112 Gold 155 Cadmium 407 Zinc .. 414 Tin 734 Iron 825 Lead 1218 Platinum 1243 Mercury 5550 By comparing this table with that of the heat- conducting powers of the same metals in the article HEAT, it will be seen that the numbers which here indicate electrical resistances are inversely proportional to those in that table which indicate heat conduc- tivities. The resistance of- fered by liquids to the pas- sage of a galvanic current can be determined with the rheostat and galvanometer in a manner similar to that for solid conductors. Plates of metal at A and B, fig. 23, are placed one above another in a vertical cylindrical vessel and connected by wires, one with a pole of a battery, and the other with the rheostat, the galvanometer being in- troduced as before. The ter- minal plates must be of metal whose relations to the fluid will not excite any electro- motive force. The best liquid conductors except mercury offer vastly greater resistance than metals. The resistance offered by dilute sulphuric acid is about 1,000,000 times that of silver, and that of water many times greater. If the strength of a series of currents passing through a wire, as measured by the tangent or the sine galvanometer, is represented by the numbers 1, 2, 3, the quan- tity of heat developed in the same time will be expressed by the numbers 1, 4, 9 ; therefore the heat generated by a galvanic current is proportional to the square of the strength of the current. With an equal strength of current the heat generated is in proportion to the ex- ternal resistance. If currents of equal strength are passed through silver and platinum wires of the same length and thickness, the latter will be heated ten times as much as the former, because the resistance offered by platinum is ten times as great as that offered by silver; but it will require more electromotive force to send the current through the platinum. Elec- trolysis. The decomposition of substances by the galvanic current when there is no con- sumption of either electrode, as in the cases we have been considering, is called electrolysis, although it differs but little from the decom- position which takes place in the cell of a bat- tery couple, the decomposed fluid in either case being, strictly speaking, an electrolyte. The first decomposition of substances by passing through them currents from the electrodes of a battery was effected by Nicholson and Car- lisle, who decomposed water in the year 1800, soon after the construction of the voltaic pile. The electrolysis of water is commonly per- formed with Faraday's voltameter, a modifica- tion of which is shown in fig. 24. Two platinum electrodes, p and n, pass through sealed orifices in the bottom of a shallow vessel, and over them are placed inverted test tubes, O and H,