Electromotive Force of different Forms of the Clark Cell. 135 



§ 17. Hermetically Sealed Cells. 



We have always felt somewhat dissatisfied with the usual practice 

 of sealiug up standard cells with marine glue or paraffin wax. Pro- 

 vided that the marine glue sealing is carefully and conscientiously 

 performed, it may, doubtless, remain good for a considerable time, if 

 the cell is not exposed to extreme variations of temperature. We 

 have not, however, ourselves succeeded in making the marine glue 

 seal stand many repetitions of the 0° to 30° C. treatment. Cells 

 which have been thus treated for a month or two have invariably 

 shown some signs of creeping. Except in extreme cases, this creep- 

 ing does not appear to produce much effect on the E.M.F. of 

 saturated cells, but in the case of unsaturated cells the effect is 

 serious. The set of twelve Carhart- Clark cells in our collection, 

 though evidently prepared and sealed with the greatest skill and 

 care, have all suffered from the creeping out of the solution in the 

 lapse of two years. One of the cells is now 5 millivolts below its 

 normal value. It is only fair to add that, owing to the extremes of 

 the Montreal climate, they have been subjected to an annual tem- 

 perature range of o° to 27° C, and that all the Muirhead cells in our 

 collection are similarly affected. 



We have succeeded in making several forms of hermetically sealed 

 cells, and we are of opinion that such cells are much to be preferred 

 as standards to those sealed in any other way. The following are 

 the principal varieties on which we have made experiments. 



(1) Cells of the H-Tyjoe with Zinc Amalgam. — We prefer to make 

 this cell in the form of an inverted Y. Fine platinum wires are 

 first sealed into the lower extremities of the inverted Y, the limbs 

 are then filled with zinc amalgam and crystals of zinc sulphate, and 

 with mercury and mercurous paste as usual. When sufficient 

 materials have been introduced, the middle leg is sealed off. This 

 inverted Y-form is much easier to make than the H-form, as it 

 involves only one T-join. In making these cells, we prefer to use 

 lead glass tubing about 5 to 8 mm. in bore. Hermetically sealed 

 cells of this form were made many years ago by Wright* for the 

 purpose of testing the effect of dissolved air on the E.M.F. 



W-Form. — When intended for immersion in a water bath, the 

 lower limbs of the inverted Y are continued upwards beyond the 

 seal to a height of 4 or 5 inches forming a W. The upturned limbs 

 are partly filled with mercury, and are used for making connections. 



As the result of our experience with cells of this description, we 

 are not inclined to recommend the use of cells containing zinc 

 amalgam at temperatures above 25° C. or below 10° C. As Lord 

 Rayleigh has observed, these cells show a very remarkable tendency 



* ' Phil. Mag.,' vol. 16 (1883), p. 28. 



