NO. 2 



HISTORY OF ELECTRIC LIGHT SCHROEDER 



73 



ing with it. He finally produced an arc in vacuum in a one-inch glass 

 tube about 50 inches long for no volts direct current circuits, which 

 was commercialized in 190 1. The tube hangs at about 15 degrees from 

 the horizontal. The lower end contains a small quantity of mercury. 

 The terminals are at each end of the tube, and the arc was first started 

 by tilting the tube by hand so that a thin stream of mercury bridged 

 the two terminals. Current immediately vaporized the mercury, 

 starting the arc. A resistance is put in series with the arc to maintain 

 the current constant on direct current constant vohage circuits. 

 Automatic starting devices were later developed, one of which con- 

 sisted of an electro-magnet that tilted the lamp, and the other of an 

 induction coil giving a high voltage which, in discharging, started the 

 arc. 



Cooper- Hew ITT [Mercury Vapor Arc Lamp, 1901. 



This gives a very efficient light, practically devoid of red but of high 

 actinic vakie, so useful in photography. 



This lamp is particularly useful in photography on account of the 

 high actinic value of its light. Its light is very diffused and is practi- 

 cally devoid of red rays, so that red objects appear black in its light. 

 The lamp consumes 3^ amperes at no volts direct current (385 

 watts) having an efficiency of about 12^ lumens per watt. 



The mercury arc is peculiar in that it acts as an electric valve 

 tending to let current flow through it only in one direction. Thus on 

 alternating current, the current impulses will readily go through it 

 in one direction, but the arc will go out in the other half cycle unless 

 means are taken to prevent this. This is accomplished by having two 

 terminals at one end of the tube, which are connected to choke coils, 

 which in turn are connected to a single coil (auto) transformer. The 

 alternating current supply mains are connected to wires tapping dif- 

 ferent parts of the coil of the auto transformer. The center of the 



