902 REPORT— 1885. 
by a long platinum spiral, the upper end of which is also hermetically sealed into- 
the glass. To the lower end of the spiral is attached a long hook, which carries 
the lower plate, and at the junction of the spiral with this hook there is a small 
globe of soft iron. It is by means of this small soft iron globe, and a magnet 
applied outside the tube, that the upper plate is lifted from the lower plate. The 
nature of the vacuum chamber and its connection to the pump will be readily. 
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PHOSPHORIC ACID 
QUADRANT ELECTROVE?ER 
SLIDE RESISTANCE 
understood from the diagram. ‘The final adjustment of distance between the two 
plates when at their nearest is done at the top of the tube by heating it and draw- 
ing out the glass carefully, or else letting it fallin till the adjustment is satisfac- 
torily made. The branch tube on the right-hand side of the diagram, which is: 
shown sealed off, is used for supplying gases to the apparatus, A small part only 
of the drying and purifying apparatus is shown in the diagram, 
For determining the value of the Volta contact effect, I have used the quad- 
rant electrometer and the compensation method of Sir William Thomson. The 
Daniell’s cell and slide resistance and the connections are shown in the figure. 
The method is so well known that it need not be described here. It was given 
in ‘ Nature,’ April 14, 1881, and has also been described in the paper of Professor 
O. Lodge, which was printed in the British Association Reports. 
The plates which I have used are zine and copper discs, a little larger than 2 
shilling. Testing them first with full air-pressure, I found that the Volta contact 
difference was rather oyer 0°74 volt: which is as nearly as possible what has been 
found by previous experimenters, and by myself experimenting a few weeks ago 
with Sir William Thomson’s old contact apparatus. 
The air was then very carefully exhausted, the exhaustion being maintained 
for two days, and finally made exceedingly good with two and a half hours work at 
the pump just before testing. The pressure during the testing was not greater than, . 
and was probably much less than =4; mm. of mercury, or 2°5 M (23 millionths of 
an atmosphere). The electrical test was then applied, and the value of the Volta 
contact effect was found to be exactly the same as before, a little over 0°74 volt. 
The final test, however, consisted in allowing the air to come back into the appa- 
ratus, and then testing electrically. This was done by breaking the point of the 
sealed tube to the right of the diagram; and as the air entered the electrometer 
test was rapidly applied. So far as the sensitiveness of the testing apparatus goes, 
there was absolutely no change in the Volta contact difference. 
My experiments, therefore, are absolutely contradictory, so far as they have 
gone, to those of Dr. W. von Zahn, quoted by Professor Lodge, who experimented 
in a high vacuum and found the Volta effect to be diminished, and to be repre- 
sented by a potential difference of only half a Daniell. 
