From the number of different properties possessed by my cells, it might
be anticipated that the different combinations of those properties would
result in cells having every variety of action. This is found to be the
case. As a general rule, the cells are noteworthy in one respect only.
Thus, if a cell is extremely sensitive to light, it may not be specially
remarkable in other respects. As a matter of fact, however, the cells
most sensitive to the light are also "U B cells."
The property of sensitiveness to light is independent of the power to
generate current by exposure to light--the best current-generating cells
being only very moderately sensitive to light, and some of the most
sensitive cells generate scarcely any current at all. Current-generating
cells are, almost without exception, "U B cells;" and the best
current-generating cells are strongly polarized, showing a considerable
change of resistance by reversing the direction of a current through
them; and they are also strong "anode cells," i.e., the surface next to
the gold offers a higher resistance to a battery current than the other
surface of the selenium does. The power to generate a current is
temporarily weakened by sending a battery current through the cell while
exposed to light, in either direction. The current generated by exposure
to light is also weakened by warming the cell, unless the cell is
arranged for producing current by exposure to heat.
The properties of sensitiveness to light and to change of battery power
are independent of each other, as I have cells which are sensitive to
change of current but absolutely insensitive to light--their resistance
remaining exactly the same whether the cells are in darkness or in
sunlight. I also have cells which are sensitive to light, but are
unaffected by change of battery power, or by reversing the direction of
the current through them.
The sensitiveness to change of battery power is also independent of the
sensitiveness to reversal of direction of the current. Among the best "L
B cells," some are "anode cells" and others are "cathode cells," while
still others are absolutely insensitive to reversal of current or to the
action of light.
_Constancy of the resistance_.--A noticeable point in my cells is the
remarkable constancy of the resistance in sunlight. Allowing for
differences in the temperature, the currents, and the light, at different
times, the resistance of a cell in sunlight will remain practically
constant during months of use and experiments, although during that time
the treatments received may have varied the resistance in dark hundreds
of thousands of ohms--sometimes carrying it up, and at others carrying it
down again, perhaps scores of times, until it is "matured," or reaches
the condition in which its resistance becomes constant.
As has already been stated, the sensitiveness of a cell to light is
increased by proper usage. This increased sensitiveness is shown, not by
a lowered resistance in light, but by an increased resistance in dark.
This change in the cells goes on, more or less rapidly, according as it
is retarded or favored by the treatment it receives, until a maximum is
reached, after which the resistance remains practically constant in both
light and dark, and the cell is then "matured," or finished. The
resistance in dark may now be 50 or even 100 times as high as when the
cell was first made, yet, whenever exposed to sunlight it promptly shows
the same resistance that it did in the beginning. The various treatments,
and even accidents, through which it has passed in the mean time, seem
not to have stirred its molecular arrangement under the action of light,
but to have expended their forces in modifying the positions which the
molecules must normally assume in darkness.
_Practical applications_.--There are many peculiarities of action
occasionally found, and the causes of such actions are not always
discernible. In practice, I have been accustomed to find the
peculiarities and weaknesses of each cell by trial, developing its
strongest properties and avoiding its weaknesses, until, when the cell is
finished, it has a definite and known character, and is fitted for
certain uses and a certain line of treatment, which should not be
departed from, as it will be at the risk of temporarily disabling it. In
consequence of the time and labor expended in making cells, in the small
way, testing, repairing damages done during experiments, etc., the cost
of the cells now is unavoidably rather high. But if made in a commercial
way, all this would be reduced to a system, and the cost would be small.
I may say here that I do not make cells for sale.
The applications or uses for these cells are almost innumerable,
embracing every branch of electrical science, especially telegraphy,
telephony, and electric lighting, but I refrain from naming them. I may
be permitted, however, to lay before you two applications, because they
are of such general scientific interest. The first is my
_Photometer_.--The light to be measured is caused to shine upon a
photo-electric current-generating cell, and the current thus produced
flows through a galvano-metric coil in circuit, whose index indicates
upon its scale the intensity of the light. The scale may be calibrated by
means of standard candles, and the deflections of the index will then
give absolute readings showing the candle power of the light being
tested. Or, the current produced by that light and that produced by the
standard candle may be compared, according to any of the known ways of
arranging and comparing different lights--the cell being lastly exposed
alternately to the two lights, to see if the index gives exactly the same
deflection with each light.
This arrangement leaves untouched the old difficulty in photometry, that
arising from the different _colors_ of different lights. I propose to
obviate that difficulty in the following manner. As is well known, gold
transmits the green rays, silver the blue rays, and so on; therefore, a
cell faced with gold will be acted upon by the green rays, one faced with
silver by the blue rays, etc. Now, if we construct three cells (or any
other number), so faced that the three, collectively, will be acted upon
by all the colors, and arrange them around the light to be tested, at
equal distances therefrom, each cell will produce a current corresponding
to the colored rays suited to it, and all together will produce a current
corresponding to all the rays emitted by the light, no matter what the
proportions of the different colors may be. The three currents may act
upon the same index, but each should have its own coil, not only for the
sake of being able to join or to isolate their influences upon the index,
but also to avoid the resistances of the other cells. If a solid
transparent conductor of electricity could be found which could be thick
enough for practical use and yet would transmit all the rays perfectly,
i.e., transmit white light unchanged, that would be still better. I have
not yet found a satisfactory conductor of that kind, but I think the plan
stated will answer the same purpose. This portion of my system I have not
practically tested, but it appears to me to give good promise of removing
the color stumbling-block, which has so long defied all efforts to remove
it, and I therefore offer it for your consideration.
_Photo-electric regulator_.--My regulator consists of a
current-generating cell arranged in front of a light, say an electric
lamp, whose light represents the varying strength of the current which
supports it. The current produced in the cell by this light flows through
an electro-magnetic apparatus by means of which mechanical movement is
produced, and this motion is utilized for changing resistances, actuating
a valve, rotating brushes, moving switches, levers, or other devices.
This has been constructed on a small scale, and operates well, and I
think it is destined to be largely used, as a most sensitive, simple, and
perfect regulator for currents, lights, dynamos, motors, etc., etc.,
whether large or small.
In conclusion, I would say that the investigation of the physical
properties of selenium still offers a rare opportunity for making very
important discoveries. But candor compels me to add that whoever
undertakes the work will find it neither an easy nor a short one. My own
experience would enable me to describe to you scores of curious
experiments and still more curious and suggestive results, but lack of
time prevents my giving more than this very incomplete outline of my
discoveries.
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