current only is required it alone is furnished, and when the maximum
force is needed the same shall be forthcoming.

[Illustration: THE CONTROLLER MAGNET.]

On the larger generators we combine with the regulator magnet above
described an exceedingly sensitive controller magnet governing the
regulation, and by whose accuracy the smallest variations of current
are counteracted, and the operation of the generator rendered perfect.
The controller magnet is contained in a box placed on the wall or
other support near the generator, and consists of a delicate double
axial magnet controlling the admission of current to the regulator,
upon the generator, and its action is exceedingly simple and
effective. So perfect is the action that in a circuit of twenty-five
to thirty lights, lights may be removed or put out in rapid succession
without apparently affecting those that remain. Besides, we have been
enabled to put out even eight or ten lights together instantly, while
the remainder burn as before. The features above set forth are
peculiar to the Thomson-Houston system, and have been thoroughly
covered by patents, and cannot therefore be adopted into other
systems.


THE THOMSON ARC LAMP.

This lamp is essentially a series lamp; that is, any number of them
can be put on one circuit wire, but a single lamp, used alone, burns
equally well. It consists of a metal frame supporting at the bottom
the holder for the globe and lower carbon, which is insulated from the
frame.