98 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
broken away in cutting down into the sulphur round the plates in order to increase 
the surface resistance. We may call the area 190 sq. centims. The micrometer screws 
were adjusted so as to leave a film space of *05 centim. This was therefore the 
thickness of the film. So much difficulty was experienced in screwing the screws back 
that it was determined for the future to use some other method of separating the 
gilded plates. During the attempts that were made to get a value of the resistance 
of the film, several facts came to light on which our subsequent practice was founded. 
The first was with respect to the necessity for insulating every part of the appa¬ 
ratus. Our connecting keys proving unfit for such high resistance work, new ones 
were made out of rods of ebonite about 20 centims. long, and were so arranged that it 
was easy to get at the ebonite in every part for the purpose of scraping it with bits 
of broken glass—a very effective way of cleaning it. It was also found that there 
was a considerable amount of surface conductivity over the sulphur, and we finally 
were obliged to use phosphorus pentoxide as a drying agent, anything less active fail¬ 
ing to give satisfactory results. The battery of test-tube storage cells was reinforced 
by a water battery"* consisting of zinc and copper plates, as described by Professor 
Rowland (‘ Phil. Mag.’ [5], vol. 23, p. 303). 
In the final experiment with the film under consideration, a voltage of 551 was 
used. It was measured—not very accurately but sufficiently so—by the simple 
means of charging a fraction of a microfarad condenser with it, and observing throws 
with a galvanometer. Another division of the microfarad was afterwards charged by 
40 Clark cells, and the throws similarly observed. 
The galvanometer employed was the one described by one of us in the ‘ Philosophical 
Magazine,’ vol. 28, which we now call the “ old ” galvanometer. It had a sensitive¬ 
ness of 1 scale division for 1*44 X 10 -n ampere, with a deflexion of about 23 scale 
* This battery is very convenient when it is once got into good order, but it is not suitable for 
work of this kind, because many days often elapse between consecutive measurements, and then, as a rule, 
a good deal has to be done to the battery to get it into good order again. For this reason we no longer 
use it. The little storage cells are also a great source of trouble and annoyance. A battery of storage 
cells, to be reliable, requires much attention and ought not to be too small. However, as this battery 
was first set up in 1887, and is still (October, 1893) in use, perhaps we ought not to complain. The result 
of our experience is that if we had to make such a battery again, we would attend to the following points : 
(1) The test-tubes might be advantageously replaced by strong glass cells—of square section, and not 
less than 3 centims. on each side—inside. (2) The lead plates should be at least 3 millims. thick. 
(3) They should be formed plates—not pasted in any way. (4) The tops of the cells and the places 
where the plates bend over should be dipped in marine glue. (5) The cells should not be crowded 
together, so that they can only be examined by being taken out. It is a mistake to save space at the 
expense of the satisfaction of this condition. (6) The plates should be separated in each celbby a sub¬ 
stantial partition of celluloid or other suitable substance, and it is better to put up with an, increased 
resistance than to cut away most of the partition for the sake of reducing it. (7) The terminal wires 
should be thick and well coated with marine glue or gutta-pei’cha. (8) The cells should be enclosed in 
a space which can be shut up, so as to prevent excessive evaporation. (9) The lead plates should hot 
approach the bottom of the cells nearer than about 6 centims. 
