22 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 



1^ 



around a wheel. On this wheel was a brake actuated by an electro- 

 magnet. The lower carbon was pulled down by an electro-magnet 

 working against a spring. When no current was flowing or when the 

 arc went out, the two carbons touched. With current on, one electro- 

 magnet set the brake and held the upper carbon stationary. The other 

 electro-magnet pulled the lower carbon down, thus striking the arc. 



None of these mechanisms regulated the length of the arc. It was 

 not until 1856 that Joseph Lacassagne and Henry Thiers, Frenchmen, 

 invented the so-called '' differential " method of control, which made 

 the carbons feed when the arc voltage, and hence length, became too 

 great. This principle was used in commercial arc lamps several 

 years afterward when they were operated on series circuits, as it had 

 the added advantage of preventing the feeding of one arc lamp affect- 



DiAGRAM OF " Differential " Method of Control of an Arc Lamp. 



This principle, invented by Lacassagne and Thiers, was used in all 

 arc lamps when they were commercially introduced on a large scale 

 more than twenty years later. 



ing another on the same circuit. This differential control consists in 

 principle of two electro-magnets, one in series with, and opposing 

 the pull of the other which is in shunt with the arc. The series magnet 

 pulls the carbons apart and strikes the arc. As the arc increases in 

 length, its voltage rises, thereby increasing the current flowing through 

 the shunt magnet. This increases the strength of the shunt magnet 

 and, when the arc becomes too long, the strength of the shunt be- 

 comes greater than that of the series magnet, thus making the carbons 

 feed. 



The actual method adopted by Lacassagne and Thiers was different 

 from this, but it had this principle. They used a column of mercury 

 on which the lower carbon floated. The upper carbon was stationary. 

 The height of the mercury column was regulated by a valve con- 



