24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



In 1857, Serrin took out his first patent on an arc lamp, the general 

 principles of which were the same as in others he made. The mechan- 

 ism consisted of two drums, one double the diameter of the other. 

 Both carbons were movable, the upper one feeding down, and the 

 lower one feeding up, being connected with chains wound around 

 the drums. The difference in consumption of the two carbons was 

 therefore compensated for by the difference in size of the drums, 

 thus maintaining the location of the arc in a fixed position. A train 



Serrin's Arc Lamp, 1857. 



This type of arc was not differentially controlled but was the first 

 commercial lamp later used. Both carbons v/ere movable, held by 

 chains wound around drums which were controlled by ratchets actu- 

 ated by an electro-magnet. 



of wheels controlled by a pawl and regulated by an electro-magnet, 

 controlled the movement of the carbons. The weight of the upper 

 carbon and its holder actuates the train of wheels. 



DEVELOPMENT OF THE DYNAMO, 184O-1860 



During the first few years after 1840 the dynamo was only a labora- 

 tory experiment. Woolrich devised a machine which had several 

 pairs of magnets and double the number of coils in order to make 

 the current obtained less pulsating. Wheatstone in 1845 patented 

 the use of electro-magnets in place of permanent magnets. Brett in 

 1848 suggested that the current, generated in the coils, be allowed to 



