Alexander Graham BELL, Ph.D.,
[Read before the American Association for the Advancement
Researches of Summer Tainter and Alexander Graham Bell
The first point to which we devoted our attention was the reduction of the resistance of crystalline selenium within manageable limits. The resistance of selenium cells, employed by former experimenters, was measured in millions of ohms, and we do not know of any record of a selenium cell measuring less than 250,000 ohms in the dark.
We have succeeded in producing sensitive selenium cells measuring only 300 ohms in the dark and 150 ohms in the light. All former experimenters seemed to have used platinum for the conducting part of their selenium cells, excepting Werner Siemens, who found that iron and copper might be employed. We have discovered that brass, although chemically acted upon by selenium, forms an excellent and convenient material ; indeed, we are inclined to believe that the chemical action between the brass and selenium has contributed to the low resistance of our cells by forming an intimate bond of union between the selenium and brass.
We have observed that melted selenium behaves to other substances as water to a greasy surface, and we are inclided to think that when selenium is used in connection with metals not chemically acted upon by it, the points of contact between the selenium and the metal offer a considerable amount of resistance to the passage of a galvanic current, and thus serve to increase the apparent resistance of the selenium.
By using brass we have been enable to construct a large number of cells of different forms. Time will only admit of my showing you two typical forms. One of these is shown in plan in fig. 1, and in section in fig. 2.
This cell consists of two brass plates insulated from one another by a sheet of mica. The upper plate has numerous perforations and brass pins attached to the lower plate, pass through these orifices so that their ends without touching the upper plate are flush with its surface.
The annular spaces between the pins and the plate are filled with selenium. The whole arrangement forms part of a galvanic circuit, and it will be observed that the current can only pass from the plate to the pins through the selenium rings.
It will also be seen that owing to the conical shape of the perforations the points of closest approximation between the pins and the plate are on the upper surface. As the effect produced by light upon selenium is chiefly a surface action, this arrangement is found to be of great advantage.
The second typical cell is cylindrical in form, for the purpose of being used with a concave reflector instead of with a lens (see fig. 3)
This cell is composed of a large number of metallic discs separated by discs of mica slightly smaller in diameter. The spaces between the brass discs over the mica are filled with selenium, and the alternate brass discs are metallically connected. The arrangement practicaly consists of a large number of annular selenium cells united in multiple arc.
The mode of applying the selenium is as follows:
The cell is heated, and when hot enough a stick of selenium is rubbed over the surface.
In order to acquire conductivity and sensitiveness the selenium must next undergo a process of annealing.
The method we first adopted was the following :
The selenium cell was placed with a thermometer in the interior of the cylindrical annealing chamber shown in fig. 4.
This was inserted in a pot of linseed oil, and the latter stood upon glass supports within another similar pot containing linseed oil. The whole arrangement was then placed over a gas stove and heated to a temperature of about 214°C., which was found to be the temperature of maximum conductivity for the selenium used.
This temperature was retained for about twenty-four hours, and the pots, with their contents, were then packed in a box so arranged as to retard radiation of heat.
The selenium took from forty to sixty hours to cool down to the temperature of the air.
A powerful battery current was passed through the selenium during the whole process of heating and cooling, in accordance with our theory that the current exerted a powerful influence in causing a set of the selenium molecules, and in retaining them in position until fixed by crystallization.
A shunted galvanometer was introduced into the circuit for the purpose of observing the changes of conductivity. We subsequently found this tedious process to be unnecessary, as during the course of our experiments we discovered a method of preparing sensitive selenium in a very few minutes.
We now simply heat the selenium over a gas stove and observe its appearance. When the selenium attains a certain temperature, the beautiful reflecting surface becomes dimmed. A cloudiness extends over it, somewhat like the film of moisture produced by breathing upon a mirror.
This appearance gradually increases and the whole surface is soon seen to be in the metallic, granular, or crystalline condition. The cell may then be taken off the stove and cooled in any suitable way. When the heating process is carried too far, the crystalline selenium is seen to melt.
Our best results have been obtained by heating the selenium until it crystallizes as stated above, and by continuing the heating until signs of melting appear, when the gas is immediately put out.
The portions that had melted instantly re-crystallize, and the selenium is found upon cooling to be a conductor, and to be sensitive to light. The whole operation occupies only a few minutes. This method has not only the advantage of being expeditious, but it proves that many of the accepted theories on this subject are fallacious.
Early experimenters considered that the selenium must be "cooled from a fused condition with extreme slowness." Later authors agree in believing that the retention of a high temperature-sbort of the fusing point-and slow cooling-are essential, and the belief is also prevalent that crystallization takes place only during the cooling process.
Our new method shows that fusion is unnecessary, that conductivity and sensitiveness can be produced without long heating and slow cooling; and that crystallization, takes place g during the heating process. We had found that on removing the source of heat, immediately on the appearance of the cloudiness above referred to, distinct and separate crystals can be observed under the microsxcope, which appear like leaden snow flakes on a ground of ruby red.
Upon removing the heat when crystallization is further advanced, we perceive under the microscope masses of these crystals arranged like basaltic columns, standing detached from one another-and at a still higher temperature the distinct columns are no longer traceable, but the whole mass resembles metallic pudding-stone with here and there a separate snow flake, like a fossil on the surface. Selenium crystals formed during slow cooling after fusion, present an entirely different appearance, showing distinct facets.
I must now endeavor to explain the means by which a beam of light can be controlled by the voice of a speaker.