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 ason arclamps. The numberis 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, 
Reynier, 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 sthem 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. ‘l'o produce this effect the carbons. 
are so arranged that a different motion is imparted to each, that 
the wearing away may be 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 are preserves its normal length and its position is 
maintained in space. This is accomplished by an ingenious 
