... In late years Photo-cells are taking a more and more
important place in Laboratories, as most precise and sensitive apparatus.
With the increase in application of photo-cells the methods of using
them are improving or, vice versa, the improvement of methods increases
the usefulness of cells.
After the development of methods of amplification of photo-electric
impulses by means of thermionic tubes, the application of cells extends
far beyond the laboratories.
We know now, after the works of G. du Prell (Ann. der Physik #3,
1923); G. Ferris (Comptes Rends November 5, 1923) and others, how
to amplify the photo-electric impulses as high as a million times
by means of a single thermionic tube.
These methods are particularly adaptable for amplification of very
weak impulses and require careful insulation and even special thermionic
tubes for best results.
In order to simplify the installation and adapt the photo-electric
cell for use of untrained operators, the following device was developed,
having in view the output sufficient to operate directly the average
mechanical relays.
This device is the combination of thermionic tube with a photo-sensitive
control electrode.
It consists of filament of oxide coated type inside of open mesh
grid enveloped completely by another grid of fine mesh.
This second grid is in electrical contact with metallic coating
of inside wall of glass container.
The fourth electrode of cylindrical shape is around the second grid.
These last three electrodes are coaxial.
The inside of the cell is coated with photo-emitting substance,
for instance alkali metal, and treated in the usual way.
Great care should be taken to prevent the alkali metal from condensing
appreciably on insulating parts of the cell.
In order to prevent the light from the filament falling on sensitive
film, the part of second grid is closed by metal shields and the filament
is operated at the temperature below visual emission.
Several connections are possible with this cell, depending on requirement
of the output.
The simplest connection is to let the second grid floating and first
grid connected to plus side of the filament and positive potential
applied to the cylinder in respect to the filament.
The cell is now operating as three electrode tubes, as the first
grid acts only for reduction of impedance of the tube and can even
be omitted.
The second grid acquires the negative charge from the electronic
flow and blocks the current between the filament and anode. If the
light will fall now on the sensitive film, it will discharge by photo-electrons
the second grid, the latter being connected with the film.
The blocking action will be reduced and part or the whole available
thermionic current will flow to the anode, depending on the rate of
discharge of film or intensity of the light.
With such arrangement, the output of the order of milliampere was
obtained, the cell being preferable of hard type.
When larger output is desired without going to the higher potentials
and without sacrificing the sensitiveness, the other circuit is preferable.
The first grid is used as an anode with low potential (order of 30
volts) due to the close spacing between this grid and filament.
The second grid is connected through the high resistance to a negative
potential with respect to the filament.
The cylinder can be connected to the first grid directly or, preferably,
have possible potential in respect to it.
The second grid is now working as outside control electrode and with
sufficient negative potential can completely block the current between
filament and first grid.
If this potential will be adjustable without light falling into the
cell, no current will flow between the electrodes and no voltage drop
across the resistance.
While illuminated the film and second grid will discharge the photoelectrons
to the cylinder and this current will produce the voltage drop across
the resistance.
The potential of the second grid will be lowered and the current
start to flow from filament to the first grid.
The amount of current depends upon rate of discharge of photo-electrons,
i.e. illumination, and charge of second grid through the resistance.
By proper choosing of spacing and mesh of second grid and adjusting
the resistance, it is possible to obtain good relation between the
intensity of the light and the output within certain limits.
The cell in the arrangement, of course, has the time lag which is
proportional to the capacity of second grid and the value of the outside
resistance.
In one tube, made as described above, the time lag with this circuit
was calculated to be of the order of 1/10000 of a second and this
has been verified experimentally up to a frequency of 3000 cycles.
This value can be considerably reduced by diminishing the capacity
of the second grid and increasing its voltage factor on the first
grid.
The continuous output obtained was of the order of five milliamperes.
This limit was due to the heat developed inside of the cell, which
distills the alkali metal on the transparent and insulating parts.
Of course by proper construction this also can be improved. For this
connection the cell, of both soft and hard type, can be used.
If the reverse order of action of the cell is desired, i.e. if it
is desirable to have the current flow while the cell is not illuminated,
the circuit can be changed as follows:
The second grid is left floating and the cylinder is made negative
with respect to the filament. The first grid will again be the main
anode.
The results with this arrangement are almost as good as obtained
with the second circuit. By means of this cell it was possible to
operate from reflected daylight or small lamp, the ordinary 150 ohms
telegraph relay which in turn operated the circuit of power line with
several amperes.
/s/ V Zworyk[i]n
