. Practical physical chemistry . herent deposit, a smallcurrent density (not exceeding 0*5 amp. per 100 sq. cm. atthe cathode) should be used. At least two electrodes shouldbe prepared in the above manner from each metal; and beforeuse, the uniformity of each set of electrodes must be tested bydetermining whether they give any when immersed inthe same solution of copper sulphate (in the case of the copperelectrodes) or of zinc sulphate (in the case of the zinc elec-trodes). Testing the Uniformity of the Electrodes.—Fix the electrodesinto tubes like the one used for the standard electrod
. Practical physical chemistry . herent deposit, a smallcurrent density (not exceeding 0*5 amp. per 100 sq. cm. atthe cathode) should be used. At least two electrodes shouldbe prepared in the above manner from each metal; and beforeuse, the uniformity of each set of electrodes must be tested bydetermining whether they give any when immersed inthe same solution of copper sulphate (in the case of the copperelectrodes) or of zinc sulphate (in the case of the zinc elec-trodes). Testing the Uniformity of the Electrodes.—Fix the electrodesinto tubes like the one used for the standard electrode (p. 230,cf. Fig. 78), and fill the tubes with a solution of copper MEASUREMENTS OF ELECTROMOTIVE FORCE 233 sulphate (in the case of the copper electrodes) and with a solu-tion of zinc sulphate (in the case of the zinc electrodes). Deci-normal solutions of the salts may be used for this connection between the electrodes is made by means ofan intermediate solution of copper sulphate or of zinc sulphate(Fig. 80).. Fig. 80. Fit up the apparatus as shown in Fig. 75, p. 224, re-placing the cell Wa by the cell Cu | solution of CuSO, | Cuor the cell Zn | solution of ZnSOi | Zn connected in series withthe standard cell Wj. By means of the two-way key, the pointof balance on the bridge wire is determined, with the cell Wjalone in the circuit, and then for the cell Wj together with oneof the above cells. If the two copper (or zinc) electrodes areuniform, the point of balance should be the same in the twocases, the cells Cu | solution of CUSO4 | Cu and Zn |solution of ZnSOi | Zn should exhibit no If, however,an of more than i millivolt should be found (for methodof calculation, see p. 226)^ then the two copper (or zinc)electrodes should be placed in a solution of copper sulphate(or zinc sulphate) and short-circuited for several hours or a 234 PRACTICAL PHYSICAL CHEMISTRY day, until they no longer give an when placed in thesame solution. Only whe
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