Selenium Cells

CHAPTER IV. Testing and Maturing Selenium Cells 32

TESTING AND MATURING SELENIUM CELLS 32 Two States. Testing and Maturing Set. Accessories for Set. Construction of Rheostat Arm for Wheatstone Bridge. Measurement of Cells by Bridge Method. Tests for Light Sensitiveness. Proper Voltage. Classification of Cells. Testing for Polarization. Treating with A. C. Raising Resistance of Cells. Testing for Current Generation. Reconstruction of Useless Cells. Measurement of Cells by Substitution Method. Sealing Cells.

CHAPTER V. Applications of Selenium Cells 51

APPLICATIONS OF SELENIUM CELLS 51 Photometric Applications. Transmission of Speech Over Beams of Light. Automatic Control of Light Buoys and Isolated Lights. As Recorder of Sunlight. Astronomical Applications. Talking Pictures. The Phonoptican, How the Blind May Read by Sound. Controlling Mechanisms at a Distance. Electric Dog. Use in Cable Telegraphy. Burglar Alarm. Selenium Batteries, Current Generators. Telephonic Properties. Effect of Different Current Sources on Sensitiveness of Cells.

CHAPTER VI. The Care of Selenium Cells 62

THE CARE OF SELENIUM CELLS 62

LIST OF ILLUSTRATIONS

FIG. PAGE 1. Bildwell Cell 7 2. Ruhmer Cell 10 3. Bell and Taintor Cell 12 4. Mercadier Cell 14 5. Gripenberg Cell 15 6. Fritts Cell 17 7. Hot Press with Cell in Place 21 8. Details of Hot Press (elevation) 22 8. Details of Hot Press (plan) 22 9. Platen for Cell; and Template for Applying Selenium 24 10. Fiber Pieces for Enclosing Cell 26 11. Mode of Assembling Cell 30 12. Testing and Maturing Set 33 13. Layout and Wiring Diagram of Testing Maturing Set 35 14. Connections for Buzzer and Induction Coil to Produce Alternating Current 36 15. Connections for Interior of Rheostat Box 38 16. Circuit for Wheatstone Bridge Measurement 40 17. Circuit for substitution Method of Measurement 47 18. Showing Cell ready for Assembly with Four Complete Cells in background 49

CHAPTER I. Selenium, the Element

SELENIUM, THE ELEMENT

Over a century ago, 1817 to be exact, the Swedish scientist Berzelius discovered a new element in the lead chambers used for the manufacture of sulphuric acid by roasting iron pyrites. Noting its resemblance to Tellurium, the name for which having been derived from the Greek for Earth, Tellus, he named the new element Selenium derived from the Greek for Moon, Selene. The ending ’um being used to indicate a metal according to the practice of naming newly discovered elements. Although believed to be a metal for many years, the chemical reaction of Selenium resembles that of sulphur to such a degree that it is now accepted to be a non-metal in its amorphous and vitreous forms. In its third or crystalline state it has many metallic characteristics and in this form termed metallic selenium. In the Periodic System it occupies the place between Tellurium and Sulphur.

Designated by the symbol Se, selenium has been found in all parts of the globe in small quantities, chiefly in combination with copper, lead and silver forming selenides, in certain pyrites and occasionally in its pure state. It was found in meteoric iron by Warren in 1909.

An idea of its wide distribution may be gained from the following table:

Mineral Composition Location Sulphur Selenide in natural sulphur Lispau Islands Eucarite Selenide of silver and copper Chili Crooksite Selenide of silver, copper and thallium Norway and Sweden Clauthalite Selenide of lead Germany Lehrbachite Selenide of lead, copper and mercury Germany Zorgite Selenide of lead and copper Germany

The element is obtained commercially as a by-product from the manufacture of sulphuric acid, various methods of extracting it from the chamber mud being employed. The usual process is to heat the well washed chamber mud with potassium cyanide and nitrate to obtain an alkaline selenate. The element is then precipitated with hydrochloric acid or sulphur dioxide.

=Selenium= exists in three well defined forms, Amorphous, Vitreous and Metallic.

=Amorphous Selenium.= This form is obtained as a finely divided brick red precipitate when sulphur dioxide is passed thru selenic acid. It is soluble in sulphuric acid and slightly so in carbon disulphide. It has a Sp. Gr. of 4.26, with no definite melting point, softening gradually and running together between 80° and 100° C. In this state it is an insulator.

=Vitreous Selenium.= When the amorphous selenium is heated to 217° C and rapidly cooled the vitreous form results. It is now a red vitreous mass, slightly less soluble in carbon bisulphide. When a thin film is held up to the light it shows blood red in color. Sp. Gr. 4.28, Atomic Weight 79.5. This form is practically an insulator having a resistance of 6 × 10⁹ ohms per Cu. Cent. at 75° C or about 3.8 × 10¹⁰ as great as that of copper. It can be electrified by friction. Vitreous selenium has no definite melting point being hard and brittle at 40° C and softening gradually as the temperature rises, becoming fluid at 210° C.

=Metallic Selenium.= By cooling melted vitreous selenium to 210° C and holding it at that temperature for a short time the metallic form results. The element is now a black glossy opaque mass, a fair conductor of electricity but improving greatly under the influence of light. It melts between 217° and 220° C, Sp. Gr. 4.788, insoluble in carbon disulphide but will dissolve in sulphuric acid to form a green solution. It is the latter form that is used in the construction of selenium cells.

When selenium is vaporized by heat it gives off dark brown fumes having an odor similar to rotting cabbage. These fumes are poisonous and care should be taken that they are not breathed to excess.

CHAPTER II. Consideration of Cell Types and Their Characteristics

CONSIDERATION OF CELL TYPES AND THEIR CHARACTERISTICS

=A selenium cell= consists essentially of two electrodes of brass or copper bridged by a thin layer of metallic selenium. When connected into a circuit with batteries and other apparatus the current flows from one electrode to the other thru this selenium bridge. Since the resistance of the selenium to an electric current depends upon the amount of light falling upon it the flow of current thru the cell will be controlled by the brilliancy of the illumination.

=Metallic selenium= being opaque, the light penetrating but ¹/₅₀,₀₀₀th of an inch as calculated by Marx, it is necessary that the selenium layer be extremely thin in order that the light may affect an appreciable proportion of the total conducting area. This condition is never reached when the electrodes lie parallel to each other with the selenium between them. However by arranging the electrodes so that the current flows at right angles to the plane of the selenium surface we can cause all the current to flow thru the light affected area. This can only be accomplished by making use of a transparent conductor for one electrode.

To realize the importance of the above factors a description of the various types of cells developed by the many investigators in this field will be of great assistance. The different workers made use of various arrangements of the electrodes but the cells fall into certain classes. These types have been named after the inventor or the one most prominent in the work on them.

=The Bildwell cell= is possibly the best known type. It is made by winding two bare wires of copper, brass, german silver or platinum on a sheet of mica or slate. The wires are spaced about ¹/₃₂nd of an inch apart. The size of the wire is of little importance, the usual practice being to use #28 wire on a form measuring two by one inches. In Fig. 1 is shown this type of construction using a mica form, the wires being fastened by passing them thru holes at the ends of the sheet.

The selenium is applied to the cell by melting it over the wires. The cell is laid on a mica covered copper plate supported over a bunsen burner. The temperature of the cell is raised to the point where a stick of selenium when touched to the cell melts. The entire surface of the cell is coated with the selenium in a very thin layer, smoothing out the lumps with a sheet of mica or a steel knife. To get a satisfactory coating the temperature must be regulated closely, if too low the selenium turns grey and the temperature must be increased to melt it, if too high the selenium collects in drops due to surface tension and is as difficult to spread as mercury. The proper state is a semi-fluid condition which it attains at 220° C when it can be easily manipulated.

When a satisfactory surface has been obtained the cell is transferred to a copper plate to cool while the bunsen burner is turned down to give a temperature of 120° C. When cool the cell is replaced on the hot copper plate and allowed to heat up again. Shortly the whole surface will turn grey in color due to the selenium crystallizing. The temperature is now slowly increased till the selenium shows signs of melting, this will be indicated by the edges turning black. The bunsen burner is immediately withdrawn and the edges allowed to recrystallize. The burner is turned down a trifle and replaced under the hot plate. The cell is watched carefully for signs of melting and if none appear it is left so for three or four hours. If it melts again the burner should be further lowered, just sufficient to keep the cell a trifle below the melting point of the selenium. The cell is then allowed to cool by lowering the burner by small amounts extending over a period of an hour. This prolonged heating and slow cooling is known as annealing.

Continue reading Selenium Cells free in the Wunder reader →