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i 
ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 103 
It was found that the capacities remained constant from day to day, 
and that the accuracy of a single determination was about 1 in 1,000, 
although the capacity to be measured was so small. 
Some mica condensers belonging to the Cavendish Laboratory were 
compared with these—details of the method will be given shortly—and it 
was found that when comparing a condenser of 1 microfarad with the 
three air condensers combined, having thus a capacity of ‘009506 micro- 
farad, so that the ratio of the two was about 100 to 1, an accuracy of 
about 1 in 1,000 was attained. It was also shown that the capacity of 
the mica condensers as thus found differed by nearly 2 per cent. from their 
values as determined by the rapid commutator, thus proving that the 
commutator method was unsuitable for a condenser showing absorption. 
Thus for three mica condensers the following values were found :— 
With commutator By slow method of comparison 
*9690 “9868 
“4934 4994 
09543 09644 
These results make the necessity for an air standard all the more 
apparent. A report on the experiments made up to that date was laid 
before the Committee at a meeting in London in April 1889. It was 
then decided to adopt Dr. Muirhead’s form of condenser, and to have 
two made on the same pattern for the Association. These have been 
constructed by the Cambridge Scientific Instrument Company, following 
Dr. Muirhead’s plan, but on an enlarged scale. Mach has a capacity of 
about ‘02 microfarad, or about six times that of one of the original 
condensers. 
Fig. 2 shows the arrangement. 
The condensers consist of twenty-four concentric tubes ; the outer tube 
is about 2 feet 9 inches high and 6 inches in diameter. Each succeeding 
tube diminishes in diameter by half an inch; the tubes are about 35 
inch in thickness, and the air space between the inside of one tube and 
the outside of the next is about °%, inch, but it was found impossible 
to get all the tubes of exactly the same thickness, so that in some cases 
the distance between the tubes is less than the above. These tubes are 
carried by two conical brass castings ; the outside surface of each casting 
forms a series of twelve steps, over which the successive tubes fit. Hach 
tube is held in position by screws. The upper cone is supported by the 
outside casing of the condenser, and twelve of the tubes hang vertically 
from it. The lower cone is carried by three ebonite pillars, about 3 inches 
in height ; the twelve tubes which are attached to it come respectively 
between those which are suspended from the upper cone. Thus the 
insulation depends on the ebonite pillars, assuming there is no leakage 
across the air from the edges of the tubes. There is an opening in the 
outer casing, closed by a door, by means of which the ebonite can be 
cleaned ; the whole is dried by placing inside a small vessel of sulphuric 
acid. In the centre of the upper cone there is a hole through which a 
rod passes. The rod is connected with the lower cone, and forms the 
electrode for the insulated cylinders. An ebonite plug, fitting tightly 
round the rod, can be pushed down so as to close the hole and prevent 
the ingress of dust when the condensers are not in use; when they are 
being used the plug is removed. 
