The armature is hung on pivots so as to be free to move only toward and from the regulating magnet on changes in the current traversing the latter, and being connected to the commutator brushes, automatically adjusts their position. By this means the power of the generator is adapted to run any number of lights within its limit of capacity, or may be short circuited purposely or by accident without difficulty arising therefrom; and a number of instances have occurred where the injurious effects of a short circuit accidentally formed have been entirely obviated by the presence of the regulator. In one instance four generators, in series representing over forty lights' capacity, were accidentally short circuited, and no injury or even noticeable action took place except a quick movement of the regulators in adapting themselves to the new conditions. Had this accident occurred to generators unprovided with regulators, great injury or possible destruction of the apparatus would have resulted. It is important to a full understanding of the regulation, to state that its action is independent of resistances introduced, that it saves power and carbons in proportion to lights extinguished, and that it compensates for speed variations above the minimum speed. The manner of its action is to control the generation of current at the source in the armature, and it does so by combining certain electrical actions so as to obtain a differential effect, such that when small force of 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.