Before entering upon this discussion, however, I would draw attention to the conditions essential to any satisfactory standard cell, as enunciated by Mr. John Henderson, D.Sc., F.R.S. E., in a recent article on the subject, which appeared in the Electrical Engineer. They are as follows: :

"(1) The standard cell must be easily made and reproduced.

(2) The E.M.F. of such a cell must remain constant under constant physical conditions.

(3) The nature of the alteration of the E.M.F. of such a cell with altered physical conditions must be accurately known.

(4) The E.M.F. must return to its original value when the original conditions are reproduced.

(5) All impurities likely to occur in the materials of which the cell is composed should produce a negligible effect upon the E.M.F."

The Rayleigh H-Type of Clark Cell is somewhat different to the Board of Trade standard mainly in its constructional form, which, as the name implies, is in the form of the letter H. A diagrammatic elevation of the cell is represented in Fig. 18, from which it will be seen that the electrodes, as also the solid and semi-solid portions of the electrolyte are placed in separate glass containing vessels communicating through a narrow glass connecting tube by means of the liquid portion of the electrolyte, instead of, as in the B.O.T. form, being superimposed in one containing vessel.

The pure zinc electrode of the B.O.T. form is replaced by an amalgam of zinc and mercury in the proportion of 10 per cent. by weight of zinc to 90 per cent. of mercury. This amalgam is surmounted by a layer of zinc sulphate crystals, whilst the mercury in the other containing tube is covered with the usual mercurous sulphate paste. The whole of the remaining space above the level of the connecting tube is filled with a saturated solution of zinc sulphate produced at 30 degs. C. The containing tubes are sealed by corks and marine glue, as in the B.O.T. form previously described, and connection is made with the electrodes by means of platinum wires fused through the glass bases of the containers.

Dr. A. Muirhead's Form of Clark Cell is a modifica

tion designed with a view to portability. On referring to the original description (Fig. 17) of the B.O.T. standard, it will be seen that, should the cell be at any time inadvertently upset or otherwise inverted, as in transit, for instance, the mercury which forms the lower electrode would tend to displace the other elements of the cell owing to its high specific gravity, and would therefore have no difficulty in reaching the zinc rod, thus shortcircuiting the cell or destroying its constan cy by amalgamation. Dr. Muirhead eliminates this difficulty by replacing the free mercury by a flat spiral of platinum wire, amalgamated in the usual manner, either by boiling in pure mercury or by immersion therein when at a red heat. The free extremity of the platinum wire is passed through the glass wall of the containing vessel and fused therein, forming one terminal of the cell. The remaining details of this cell are practically identical with those of the B.O.T. form.

Professor Carhart's Form of Clark Cell is very similar to the B.O.T. form, but differs somewhat from it in detail. It is represented in Fig. 19. Connection is made with the mercury by means of a platinum wire fused directly through the glass. The paste is made up, as usual, and placed on the surface of the mercury. Over

this, again, is a layer of asbestos which effectually prevents contact between the zinc and mercury, whilst the former is flattened out at its lower or active extremity into the form shown in the figure, in order to increase its active surface, and is protected above from the effects of local action, which are often apparent in the ordinary form, by an encircling glass sleeve.

In its chemical synthesis this cell differs from the B.O.T. form in that the zinc sulphate solution is saturated at 0 C., instead of 30°C., and, in consequence, its resultant E.M.F. is some 0.4 per cent. higher than that of the B.O.T. form, being 1.438 standard volts @ 15° C

Professor Callendar and Barnes' "Inverted" Form of Clark Cell is specially suited for delicate work in that it is constructed with internal electrodes in a containing vessel of small diameter, thus admitting of its total immersion in a water bath for exact temperature determinations, and also from its consequently small thermal co-efficient, a rapid assumption of an even temperature on the part of the entire cell and its contents. The cell is represented in Fig. 20, from which it will be seen that the zinc electrode, as in the Rayleigh H-form of cell, is replaced by an amalgam of zinc and mercury,

Marine Clue

Cork

Zinc Sulphate Sol.

Mercurous Sulph. Pasto

Zine Subhate

Zinc Amalgam

FIG. 20.

whilst the mercury appears in the form of a flattened, amalgamated platinum wire. Both these electrodes are connected by long platinum wires sealed in glass tubes. with the exterior of the apparatus, the actual connection being made by means of mercury cups formed by contracting the bore of the leading-in tubes around the wires at their upper extremities. As will be seen from the figure, the actual sealing of the cell is effected some half-way down the outer containing tube, which can, in consequence, be immersed in a water bath for practically its entire length, and, owing to its small diameter, it will speedily attain the temperature of that bath.

R. Wachsmuth and W. Faeger have constructed a cell similar to the Rayleigh H-form, in which an amalgam of cadmium, 1 part by weight of cadmium to 6 of mercury, replaces the zinc amalgam, and is surmounted by a layer of cadmium sulphate crystals. The mercury in the other tube remains the same, being covered with the usual mercurous sulphate, but the liquid electrolyte with which the cell is filled consists of a saturated solution of cadmium sulphate. The E.M.F. of this cell at a temperature of 20 degs. C. is 1.019 volt, whilst its temperature co-efficient is very much lower than that of the Latimer Clark cell pure and simple.

The above described inverted cell of Messrs. Callendar and Barnes can also be constructed with cadmium in the place of the zinc, forming portion of the electrode and electrolyte.

The well-known Daniell cell has also been adopted as a standard. A cell of this type for laboratory use is to be met with in Dr. Fleming's Standard Cell, which is illustrated in Fig. 21, and consists of a U-tube A B, mounted vertically on a wooden stand. The two limbs A and B of the U-tube are open at their upper extremities for the reception of stoppers carrying the electrodes Zn. and Cu., which consist respectively of chemically pure zinc amalgamated with mercury, and electro-deposited copper. When the cell is out of use the electrodes are removed from the main limbs of the tube and placed in the subsidiary tubes a b, which serve as containing racks until the cell is again required for use, when they are replaced in their former position. Just below the upper extremities of the limbs A B, two

branch tubes emerge, the passages of which are controlled by suitable cocks; these branch tubes are surmounted by spherical glass reservoirs C D, the office of

which will be alluded to shortly. A branch drainage tube E, with control clock, is taken from the bend of the U-tube, and communicates with an independent vessel F below, whilst a second similarly controlled branch H is taken from the right hand limb at the point shown, and also communicates with the same vessel F. This latter branch determines the dividing line between the two liquids forming the electrolyte, which are respectively sulphate of zinc and sulphate of copper solutions, in the limbs A and B. The reservoirs C and D contain a further supply of the corresponding solutions, which is drawn upon to replace any which may be drained off at either of the branches E or H.

As the reader will be already aware, it is necessary, when dealing with Daniell cells of the gravity" type, of which this cell constitutes an example, to preserve a well-defined line of demarcation between the two dissimilar liquids which compose the electrolyte in order to prevent either liquid from reaching the opposite elec