784 
LORD RAYLEIGH OR THE CLARK CELL AS 
It maybe convenient to recall that cells 1, 5, were made in Oct., 1883; 10-13 
and 14-19 in May, 1884 ; H 5 in March, 1884 ; H 10 , H n also in March, 1884. 
§ 41. In cells of the ordinary type the principal source of weakness is im¬ 
perfect sealing at the top, due to cracks in the paraffin wax. As pointed out by Dr. 
Alder Wright,* a better result is obtained if the whole cell be imbedded in a large 
mass of wax than when (as in my cells) the wax is applied merely inside the tube, 
above the cork sustaining the zinc. During the last year I have replaced paraffin 
by marine glue, which, so far as can be judged at present, may be relied upon to 
effect a complete seal. The procedure will be described presently more in detail. 
The cause of failure in the H-cells is of a different nature. Many of the earlier 
cells had been found to break in the amakam leg, and the trouble was attributed to a 
hardening and expansion of the contents (§ 29). Such a hardening had, in fact, been 
observed in one or two cases. More recent experience, however, has proved that the 
cause must be looked for elsewhere, several cells having failed in which no trace of 
solid amalgam was to be found. Nevertheless the amalgam is the cause of the 
trouble, for out of a large number of breakages not one has occurred in the leg con¬ 
taining pure mercury. It would appear that some alloying takes place with the 
platinum wire in contact with the amalgam, and that this gradually extends itself 
with fatal results to the paid of the platinum sealed into the glass, from which 
place the cracks are always observed to radiate. It is hoped that a cure will be 
found in a plan, adopted for some recent cells, of melting in a little cement (marine 
glue has been used) so as to protect from the amalgam the part of the platinum 
which lies nearest to the glass ; but it is too soon to speak with certainty. 
§ 42. The H-form lends itself to hermetical sealing, and at one time I antici¬ 
pated advantage from this course. There is, however, such a large amount of 
spare liquid that there is no likelihood of trouble from desiccation, even if the 
corks allow a little evaporation. Indeed, by withdrawing the corks a fresh supply 
of liquid could be introduced at any time. It happened on one occasion that an 
H-cell to which a large excess of salt had been added, was so far crusted up 
next the metallic surfaces that it began to show signs of failing E.M.F., much as 
if it were going dry. The mass was so compact that no impression could be 
made upon it with a glass rod ; but it was bored through with a steel reamer, 
when the E.M.F. at once recovered its normal value. In such cases the acces¬ 
sibility is advantageous, especially for purposes of experiment. It is well, however, 
to avoid such a large excess of salt as was present in this case. By alternate 
melting and crystallisation as the temperature rises and falls, there is a tendency 
to aggregation, of which the cell above referred to affords an extreme example. 
In the construction of cells with solid zinc electrodes, I have fallen back upon 
a simplified pattern—nothing more in fact than a small tube with a platinum 
wire sealed through its closed end. See figure. 
* Pliil. Mag., July, 1883, p. 32. 
