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platinum, and is arranged so as to lie half in the electrolyte, which fills the lower part of A, and half in the gas. The gas escapes through the mercury trap H which prevents air from getting backwards into the apparatus. The electrode vessel is

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in communication through B with the beaker D, which is filled with the same electrolyte, and serves to receive the other electrode. A funnel is provided, with a tap, G, for convenience of washing out the end of the gas electrode. The

whole arrangement can be sunk in a thermostat. For successful working it is desirable that there should be but little electrolyte in A, so that it may be the more quickly saturated with gas. To keep the gas from going through by jerks, a glass tube drawn out very fine, to serve as a resistance, may be inserted between K and A.

Wilsmore finds that such an electrode made up with hydrogen in normal sulphuric acid, reaches a constant value in about a quarter of an hour, and can be relied on to nearly the same extent as the calomel electrode, i.e. to one or two tenths of a millivolt. It gives, with the Hg: Hg,SO, electrode, also in normal acid, an electromotive force o°6953 at 25°, with the normal calomel electrode o'3276, mercury being positive, in both cases.

From the mean of several combinations Wilsmore concludes that H in normal ionic solution of hydrogen ions is o'283 negative to the mercury of the normal calomel electrode, and therefore, taking the absolute potential of the latter as +0560, the absolute potential of hydrogen in such a solution iso277. This is the number quoted in the table of electroaffinities (p. 159). There is, however, much uncertainty in the reduction to normal ionic concentration, so that for practical purposes it is better to take the electrode actually used for standard-preferably the normal calomel electrode.

For further particulars as to the use of the gas electrode reference may be made to the work of Emil Bose.1

Another device used by Wilsmore is to polarise a platinum. point cathodically with a definite E.M.F., and use it as a potential standard. The electrode in question should be very small (a wire sealed through glass and cut off short), but well platinised; it is used as cathode, with a large platinum anode, in normal sulphuric acid, 1°2 to 13 volts being applied to the combination. The point is then slightly polarised, and may serve as a standard electrode for the measurement of any other electrode immersed in the same solution. The potential of the point is as follows :—

Zeitschr. phys. Chem., 34. 754 (1900).

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A standard, or normal, cell is one which, if made up with due care, can be relied upon to possess a definite and permanent electromotive force, so that it can be used as a standard in electrical measurements. Since all cells, when discharging, suffer a certain amount of concentration, if not also chemical polarisation, and since on account of resistance the potential difference between the terminals is reduced during discharge, it is evident that a cell can only be a completely satisfactory standard when used on open circuit. Standard cells are therefore normally used with the potentiometer: if employed to give current, it must be only minute compared with what the cell could give on short-circuiting. Thus if an accumulator (with acid of measured density) be designed to gives ten amperes, then ampere may be regarded as a minute current for it; it will scarcely polarise at all when giving this small current, and may be regarded as a rough standard (reliable to 1 per cent.) when so used. But a Clark cell, which would perhaps only give ampere when short circuited, could not be treated as a good standard if yielding more than 100000 ampere.

The conditions for a satisfactory standard cell are-—

(i.) It must be made of materials so well defined chemically that it can be reproduced with exactness.

(ii.) When brought back to the same external conditions (especially temperature), it must give the same electromotive force.

1 See the monograph by Jaeger, Die Normal-Elemente (Knapp: Hallé, 1902).

T. P. C.

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(iii.) When current is taken out of or put into the cell, though it may suffer temporary changes, it must recover completely, so that after repeated use the electromotive force shall be unaltered.

A cell such as the Daniell as commonly employed is an imperfect standard, even if made with absolutely definite chemical materials, because when current is taken out of it the concentration of the zinc sulphate is permanently increased, that of the copper sulphate permanently decreased; the electromotive force would therefore suffer a steady change (decrease) with lapse of time. To avoid such changes it is necessary that both solutions should be saturated, and be maintained so by a supply of the solid salts. Hence the combination

Zn ZnSO,.7H.O crystals: ZnSO, solution: CuSO, solution: CuSO4.5H2O crystals: Cu

But

may serve, so far as the above conditions are concerned. the permanence of the cell is affected by another consideration.

(iv.) The materials of the cell must constitute a system instable chemical equilibrium; or if this is not possible, the diffusive processes that occur must be extremely slow.

The Daniell cell does not constitute a system in equilibrium, for if the zinc rod be allowed to touch the copper sulphate solution, copper will be deposited on the zinc, and the electromotive force altered. Neither is the diffusive process slow enough in practice, for copper sulphate, being easily soluble, is present in large quantities in the electrolyte, and easily diffuses into the part of the cell containing the zinc. If a porous jar be used to keep the two liquids apart, it will only serve for a few days, or at best weeks.

No cell has been invented which satisfies condition (iv.) completely, and it is hardly conceivable that a completely stable system could be found to form a voltaic cell. But great practical improvement can be made on the Daniell in two ways: (a) by using a nearly insoluble salt, such as Hg2SO or AgCl as depolariser; (b) by using in place of copper, mercury, because then the traces of mercury that, by diffusion, reach the active metal (say zinc) only serve to amalgamate it further, and

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if it is already amalgamated, this makes an inconsiderable difference to the electromotive force.

Both these points are satisfied in the Clark and cadmium cells, which are by far the most important and practically useful cells; some others that have from time to time been used may be included in a list, as follows::

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Zn ZnCl,

Hg,Cl2: Hg (Helmholtz, Ostwald)

Cd CdSO: Hg2SO: Hg (Weston)

In the choice of a depolariser the least soluble is not, despite its less diffusion, always to be preferred; in practice the above cells with mercurous sulphate are found more satisfactory than those with the much more insoluble chlorides, because the latter polarise more readily. When current is taken out of a Clark cell, it is conveyed to the cathode by discharge of mercuro-ions; the number of these in solution is therefore temporarily diminished. The supply is, of course, made up by further solution of the solid mercurous salt, but as this takes time, there is temporary polarisation. Much more so is this the case in a chloride or oxide cell (Helmholtz or Gouy), where the stock of mercuro-ions in solution is perhaps a thousand times smaller. In working a potentiometer it is unavoidable to pass small currents through the standard cell before the final adjustment is arrived at; hence we arrive at another condition :

(v.) The depolariser must not be so very insoluble as to make the cell excessively sensitive to polarisation.

The Clark cell was for long regarded as the best standard, but the cadmium cell possesses, as compared with it, the important advantage of a practically negligible temperature coefficient. It is therefore coming to replace the Clark as standard, and we shall first describe its construction in detail; most of the directions may, however, be regarded as applying

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