78 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



starting electrode. It then goes to the pool of mercury, through the 

 reactance and through the series circuit. The other half cycle of 

 alternating current goes to the other arm of the rectifier tuhe, through 

 the mercury vapor, etc., and through the series circuit. Thus a pulsat- 

 ing direct current flows through the series circuit, the magnetic action 

 of the reactance coil making the pulsations of current overlap each 

 other, which prevents the mercury arc from going out. 



Eaklv Mercury Arc Rixtu-'iek Installation. 



INCANDESCENT LAMP DEVELOPMENTS, 1894-I904 



With the development of a waterproof base in 1900, by the use of 

 a waterproof cement instead of plaster of Paris to fasten the base to 

 the bulb, porcelain at first and later glass being used to insulate the 

 terminals of the base from each other, lamps could be exposed to the 

 weather and give good results. Electric sign lighting therefore re- 

 ceived a great stimulus, and lamps as low as 2 candlepower for no 

 volts were designed for this purpose. Carbon lamps with concentrated 

 filaments were also made for stereoptican and other focussing pur- 

 poses. These lamps were made in sizes from 20 to 100 candlepower. 

 The arc lamp was more desirable for larger units. 



