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
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.