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THE THERMIONIC PHOTO-ELECTRIC CELL

E&MP 94.006

Photo Electric Cells

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... 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

 

Original Caption by Science Service
© Westinghouse Elec. & Mfg. Co.



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