Electricity for public schools and colleges . whose principle is explained inChapter VI. § 14. But far more satisfactory results can be obtained by use of any form ofquadrant electrometer (say the Elliott form of Chapter X. § 33), in whichone pair of quadrants are to earth ; such an instrument indicates quantitativeresults with an accuracy sufficient for lecture experiments. (i.) The figure represents a simple experiment performed with the con- 166 ELECTRICITY CH. XI. densing gold-leaf electroscope and a compound bar composed of a piece of zincand a piece of copper soldered end to end. We may
Electricity for public schools and colleges . whose principle is explained inChapter VI. § 14. But far more satisfactory results can be obtained by use of any form ofquadrant electrometer (say the Elliott form of Chapter X. § 33), in whichone pair of quadrants are to earth ; such an instrument indicates quantitativeresults with an accuracy sufficient for lecture experiments. (i.) The figure represents a simple experiment performed with the con- 166 ELECTRICITY CH. XI. densing gold-leaf electroscope and a compound bar composed of a piece of zincand a piece of copper soldered end to end. We may use the quadrant electrometer, putting one pair of cjuadrants toearth, and connecting the other pair with an insulated terminal. If the zinc be held in the hand, while we touch the lower condensing plateof the gold-leaf electroscope (or terminal of the electrometer) with the copper,it will be found that the condensing plate (or insulated pair of quadrants) isnow at a — potential. (For use of electroscope and electrometer see Chapter X.). Fig. i. In this experiment it is assumed as sufficiently proved by various con-vergent pieces of evidence that the zinc, held in the moistened hand, ispractically at the zero-V of the earth ; and that the contact of the copper withthe brass of the electroscope gives no A V. Hence, it is argued, the electroscope or electrometer indicates the A V dueto the contact of the zinc and copper alone, and shows that zinc is -I- tocopper. (ii.) An electroscope is provided with an upper condensing plate made ofzinc. This is, as usual, provided on its under surface with an insulating layerof lac varnish. Both plates are as usual carefully discharged by means of aBunsens flame, until no movement of the leaves is observed on raising orlowering the upper plate. CH, XI. THE PHENOMENA OF ELECTRIC CURRENTS 16/
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