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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



A very simple device was used to automatically switch the arc from 

 one to the other pair of carbons, when the first pair was consumed. 

 This device consisted of a triangular-shaped piece of iron connected 

 to the solenoid controlling the arc. There was a groove on each of the 

 outer two corners of this triangle, one groove wider than the other. 

 An iron washer surrounded each upper carbon. The edge of each 

 washer rested in a groove. The washer in the narrow groove made a 

 comparatively tight fit about its carbon. The other washer in the 

 wider groove had a loose fit about its carbon. Pins prevented the 

 washer from falling below given points. Both pairs of carbons 



Thomson-Houston Arc Dynamo, 1878. 



This dynamo was standard for many years. This machine is in the 

 collection of the Smithsonian Institution. 



touched each other at the start. When current was turned on, the 

 solenoid lifted the triangle, the loose-fitting washer gripped its carbon 

 first, so that current then only passed through the other pair of carbons 

 which were still touching each other. The further movement of the 

 solenoid then separated these carbons, the arc starting between them. 

 When this pair of carbons became consumed, they could not feed any 

 more so that the solenoid would then allow the other pair of carbons 

 to touch, transferring the arc to that pair. 



Elihu Thomson and Edwin J. Houston in 1878 made a very success- 

 ful and complete arc light system. Their dynamo was specially 



