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Selenium | Battery | Resistance in | Resistance in |
cell. | power. | dark. | sunlight. | Ratio.
----------+-----------+---------------+---------------+----------
| | ohms. | ohms. |
No. 22 |5 elements.| 39,000 | 340 |114 to 1
" 23[6]|5 " | 14,000 | 170 | 82.3 " "
" 24[7]|5 " | 648,000 | 2,400 |270 " "
" 25 |5 " | 180,000 | 930 |196.5 " "
" 26 |5 " | 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 " "
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[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 affect the
_sensitiveness of selenium to light_.
"If one kind of battery current destroys its sensitiveness, may we not
suppose that another kind might increase its sensitiveness? Although the
Leclanche has operated well, some other may operate still better, and by
its special fitness for use on selenium cells may intensify their
actions, and so bring to light other properties yet unthought of. Is not
here a promising field for experiment, in testing the various forms of
battery already known, or even devising some new form especially adapted
to the needs and peculiarities of selenium cells?"
One year ago I made the foregoing suggestion in a paper on _A New Form of
Selenium Cell_, presented before this Association at Minneapolis. I am
now at liberty to state that my photo-electric battery, presently to be
described, marks an advance in the direction indicated. The current from
this battery increases the sensitiveness of the cells to light, and also
to reversal of current. One cell whose highest ratio in light was about
83 to 1, with the Leclanche battery, when measured with my battery gave a
ratio of 120 to 1. It seems to make the resistance of the cell both
higher in dark and lower in sunlight than with the Leclanche battery. But
the field is yet open to others, for the discovery of a battery which may
be still better for use with selenium cells.
3. _The two surfaces of the selenium act differently toward currents_
sent into them from the contiguous conductors. One surface offers a
higher resistance to the current than the other. The former I utilize as
the anode surface, as I have found that the cell is more sensitive to
light when the current enters at that surface, which is ordinarily the
one covered by the gold or other transparent conductor. Some cells have
this property but feebly developed; but in one instance the resistance
offered to the current by the anode surface was 256 times as high as that
offered by the cathode surface to the same current. In the majority of
cases, however, the ratio does not exceed ten times. Table B gives some
recent results.