100 PROFESSOR R. THRELFALL AND MR, J. H. D. BREARLEY 
The next two series of experiments failed, owing to the spots on the plates 
becoming more and more marked, and though some of the results are probably 
unvitiated by this source of error, we have decided to leave them on one side. So 
far as they went they were very variable, the variability probably depending on the 
action of the current itself and on the time. To remove the source of uncertainty, 
due to the perforation of the gold film, we reluctantly decided to prepare two new 
plates, and after some difficulty obtained two brass castings, which appeared to be 
perfect after they were shaped. These were then gilt very heavily—several ounces 
of gold must have been deposited on them—and the gilding, burnishing, and regilding 
were carried out as before. A preliminary experiment was then made to find if the 
sulphur had any action, when it turned out that though the spots were fewer and 
smaller than before, they were still present. This induced us to put the plates on one 
side, and some thick aluminium having by this time come to hand, two new plates 
were made of aluminium. These plates have remained in use ever since. They were 
got flat on the surface plate, and we trusted to keeping them apart by bits of quartz 
thread instead of screws, a process that turned out quite satisfactory. A large 
number of experiments were made on bright and scraped pieces of aluminium, with 
the object of finding out whether sulphur had any action on the plates during the 
time when and at the temperature at which they were exposed to its action. No 
trace of action having been discovered, even when the sulphur was burned on the 
plates, it was decided that they could be used with confidence. The upper plate was 
provided with an aluminium handle attached to it by aluminium screws ; the handle 
was always removed during the electrical measurements. The dimensions of the 
plates were obtained by a series of measurements with the vernier callipers, which 
will be set down here, as the other absolute measurements depend upon them. These 
callipers had been found on a previous occasion to agree with the standard metre 
within very narrow limits. 
Top Plate. 
Measurement in terms of the scale of brass callipers by Elliott Bros. Corners 
named A, 
B. C, D, in order. 
Temperature, 24-8° C. 
Side A-B. 
Side B-C. 
centims. 
centims. 
At one end . 
12-361 
At one end . 
12-653 
At middle . 
12-364 
At middle . 
12-652 
At other end 
12-363 
At other end . 
12-651 
Mean 
12-3626 
Mean 
12-652 
