W.
Gryllis Adams, of King's College, London; Dr. Werner Siemens has No. 25,
and Prof. George F. Barker, of Philadelphia, has No. 26.]
[Footnote 7: No. 24 was measured with a bridge multiplier of 6 to 1.]
Cells which are sensitive to light improve by being used daily, and their
sensitiveness becomes less if they are laid aside and not used for a
considerable length of time, especially if allowed to become overheated.
They should be kept cool, and exposed to light frequently, whether they
are used or not.
_Mode of measuring cells_.--So great is the sensitiveness of these cells
to external influences, that it is necessary to adopt some particular
system in measuring their resistance and to adhere strictly to that
system, as every change in the method of measurement produces a
difference in the result, and the different measurements would not be
comparable with each other. The reason for this will be explained
presently.
The system I have adopted is the Wheatstone's bridge arrangement, with
equal sides, never using multipliers except for some experimental
purpose. In each multiplier wire I have 500 ohms resistance. When the
bridge is balanced, one-half of the current flows through the cell and
acts upon the selenium.
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