106 TRANSACTIONS OF THE [MAR. 9^ 



condition is best fulfilled, for the production of the arc light,, 

 when two pieces of carbon are made to touch and then slightly 

 separate ; the current in passing between the space so increases 

 the resistance as to produce light of great intensity, and is called 

 the arc light. 



That a constant resistance may be obtained favorable to the 

 production of light, some mechanical device that will separate 

 and regulate the distance of the carbons is essential. Such an 

 apparatus is called a regulator, or arc lamp. 



There are few inventions on which so much ingenuity has 

 been expended as on arc lamps. The number is already very great 

 and is on the increase, and every bulletin of the Patent Office 

 is sure to contain several inventions of this kind. 



The first lamp for regulating the position of the carbons was 

 devised by Thomas Wright, of London, in 1845. 



A few years later Le Molt, of France, devised a clockwork 

 regulator which kept the light steady for several minutes at a 

 time, but the flickering was too great to make it of any practical 

 value. 



These were followed by Archereaus, Jasper, lurgenseh, Gaiffe, 

 Keynier, Foucault, and many others, each lamp an improvement 

 on the older forms, which brought the standard of regulators to 

 its present state of perfection. Flickering was regarded at 

 one time as a part of an arc light, just as sparking at the 

 brushes of a dynamo was thought to be a necessary accompani- 

 ment. 



In 1849 Foucault devised a clockwork regulator that. differed 

 in many details from the ones before used and gave comparative 

 satisfaction. It was extensively employed for scientific purposes, 

 and, in fact, was the only lamp that was suitable for lantern 

 projection, and is used to-day in many physical laboratories in 

 preference to other lamps. 



The next advance was by Serrin, whose regulator kept the 

 carbons exceedingly steady and maintained them in their rela- 

 tive positions. This lamp has been used for lighthouse illumi- 

 nation, and still maintains that position; for, like the Foucault, 

 it is a focussing lamp, the function of which is to maintain the 

 carbons at a central point. To produce this effect the carbons 

 are so arranged that a different motion is imparted to each, that 

 the wearing away may bo compensated for by a more rapid 

 movement of the positive pole with only a gradual waste of the 

 negative carbon. By this simultaneous approach of both car- 

 bons, the positive moving through double the distance of the 

 negative, the arc preserves its normal length and its position is 

 maintained in space. This is accomplished by an ingenious 



