In 1879 Edison's laboratory had developed the means to evacuate glass lamp globes to less than a mTorr in twenty minutes.@footnote 1@ Among Edison's nearly 1100 patents are five for vacuum pump improvements and at least nine others that are vacuum-related; most were applied for in 1880-1881. Edison had hired Ludwig Boehm, who had worked for Geissler, to construct Geissler pumps and "mercury drop" pumps based on those developed by Crookes and Sprengler and used in experiments described by de la Rue and Muller (France) in 1878. The Sprengler pump required a continuous gravity-fed stream of mercury droplets to trap, compress, and exhaust the air. Edison patents describe means to automate the delivery of the mercury. Other patents described the vacuum technology that Edison investigated and employed. Various means were used to remove residual gases: driving adsorbed water from glass by direct heating, and using iron particles or incandescent iron "to decompose moisture," phosphorous pentoxide and charcoal to absorb moisture, halogen or a halide to remove mercury vapor, metals such as copper or zinc to remove chlorine, and "magnesia, charcoal, and other inert substances which absorb gases in their pores." Pressure was measured with a McLeod gauge, and a high (hard) vacuum was indicated when the glow discharge in a Geissler tube was extinguished. Edison also patented a method for preserving food by sealing in an evacuated glass globe (but omitted the essential bacteria-killing heating step). A vacuum was also required for the evaporation and sputter-coating processes that later were used to produce molds for the manufacture of phonograph cylinders.@footnote 2@ @FootnoteText@ @footnote 1@R. Friedel, P. Israel and B.S. Flinn, "Edison's Electric Light: Biography of an Invention," Rutgers Univ. Press (1987), pp. 61-62. @footnote 2@ R.K. Waits, J. Vac. Sci. Technol. A 19(4), Jul/Aug 2001.