135,000 | 710 |190 " "
" 107 |5 " | 118,000 | 740 |159 " "
" 108 |5 " | 200,000 | 900 |222 " "
" 122 |5 " | 56,000 | 220 |254.5 " "
" 129[6]|5 " | 200,000 | 940 |212 " "
" 137 |5 " | 108,000 | 320 |337.5 " "
-----------------------------------------------------------------

[Footnote 6: Cells No. 23 and No. 129 are now in possession of Prof. 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. Between the bridge and the cell is a reversing
switch, so that the current can be reversed through the cell without
changing its course through the bridge. A Bradley tangent galvanometer is
used, employing the coil of 160 ohms resistance. The Leclanche battery is
exclusively used in measurements for comparison.

2. _The kind of battery employed_ has a marked effect upon the
sensitiveness to light, which is largely reduced or entirely destroyed
when the bichromate battery is used. The same cells again become
extremely sensitive with the Leclanche battery. We might expect that a
change in the current employed would cause a change in the _resistance_
of a cell, but it is not clear how or why it should affe