A use of the photo-electric ceil already known to the public through crime pictures was also illustrated. This was the outdoor and indoor burglar alarm systems, using invisible infra-red rays and employing, in the case of the indoor systems, mirrors to extend the field of protection. The light projectors used were fitted with special infrared glass filters, and were suitable for distances up to 100 fect, and even more if.no external light could influence the photocell Even mir-tors forming part of the decoration scheme of a room could be used if desired. speed control to guard against wear and tear on light lead; automatic control of doors to guard against dust and drafts, and to facilitate the passage of vehicles through folding or sliding doors; automatic control of traffic signals; and machinery failure or danger alarms, water and other pressure gauge alarms, and other safety devices including the guarding of prison walls, Other uses to which this marvellous ray could be put were the switching on of lights in locked shops, banks, warehouses, etc, by directing the beam of a torch on the cell, so that the premises might be scrutinized from the outside; the protection of lift doors to ensure that they remained open as long as passengers were entering or leaving; the detection of poisonous gases in mines or ships' holds; smoke detection to give an alarm when the smoke in certain processes reached a predetermined density, or to produce a periodic or continuous chart showing the extent of atmos-phene solution; control of paper making machines to regulate the thickness, and to guard against damage and waste of time due to breakages during the rolling process: control of gas furnaces to shut off the gas automatically in the event of the flame being extirizuished; stunt advertising by shop windows or novelty demonstrations and control of illuminated signs; sound reproduction for talking films; phototelegraphy, astronomy, television, photometry, and many r purposes.
Text read from the printed page and cleaned; old type leaves the occasional error.