TRANSACTIONS OF SECTION A. 591 



8. On a Simple Modification of the Board of Trade Form of the Standard 

 Clark Cell. By H. L. Callendar, M.A., F.R.S., Professor of 

 Physics, and H. T. Barxks, M.A.Sc, Demonstrator oj Physics, of 

 McGill University, Montreal. 



The authors have been engaged for some years past in experiments on the 

 variation of E.M.F. of the Clark cell under the most exacting conditions of tem- 

 perature change. They have studied the behaviour of various types of the cell 

 and have recently devised a very simple form, in many respects closely resemblino- 

 that described in the Board of Trade Memorandum, but somevrhat easier to con- 

 struct, and also entirely free from diffusion-lag in the changes of its E.M.F. 

 consequent on the most sudden variations of temperature. 



The cell is set up in a test-tube, but the materials are tilled in the inverse of 

 the usual order. First zinc amalgam, to which connection is made by means of a 

 •wire sealed into a glass tube. Next a layer of crystals of zinc sulphate, followed 

 by a layer of paste of mercurous sulphate prepared in the usual manner, in which 

 is coiled a fine amalgamated platinum wire, which serves in place of the mercury. 

 The whole is sealed either with a cork and marine glue, or better hermetically, by 

 sealing the glass tube on to the platinum electrodes. 



With this method of construction, both the elements are always in contact 

 with crystals, and there can be no diffusion-lag. The cells are at least equal to 

 the H form in this respect, and are much easier to make, and more convenient to 

 use, especially for immersion in a water-bath. 



The same method of construction has also been applied with success to the 

 cadmium cell. These cells appear to be as reliable as the Clark cells at tempera- 

 tures above 10° C, but the E.M.F. is dependent upon the proportions of the amal- 

 gam. Below 10° C, there appear to be two possible rates of variation of the E.M.F., 

 corresponding to different hydrates, as shown in a recent communication to the 

 Royal Society to be the case with the Clark cell between the temperatui-es ',iO° 

 and 50°, 



9. On the Cyclical Variation with Temperature of the F.M.F. of the 

 H Form of Clark's Cell. By F. S. Spiers, F. Twyman, and W, L. 

 Watkrs, 



10. On the Disruptive Discharge in Air and Dielectric Liquids. 

 By T, W. Edmondson. 



The object of the experiments described was to determine, if possible, the 

 relation existing between the spark-length and the potential at which the dis- 

 ruptive discharge takes place in air and a number of insulating oils, when the 

 electrodes used are spheres. The measurements in air wea-e made by direct 

 readings of a guard-ring electrometer and a spark micrometer, connected in parallel 

 with a Wimshurst machine. 



The curves for air, in which the ordinates represent potential differences, and 

 the abscissas the corresponding spark-lengths, are found to be hyperbolic and ai-e 

 represented by equations of the form — 



Y" = ad + bcP, 



where V is given in C.G.S. units and d in millimetres. 

 The values of a and h obtained were — 



Diam. of Spheres in Cm. a t 



•5 2.35-14 83-25 



1-0 lSG-36 99-43 



2-0 144-41 114-49 



3-0 49-42 144-71 



The differences between the calculated and observed values of V are in 

 general, not more than 1 per cent., and there is also good agreement with the 

 results previously obtained by Bailie, Bichat and Blondlot, Paschen and Freyberg. 



