The Thomson Regulating Gear

Later on we will show that pushing the brushes together or pulling them apart alter the strength of the current, but for the present just accept the fact and we will show how the brushes are varied. It is accomplished by the mechanism shown in Fig. 18. The brushes are fixed to the levers YY and Y₂Y₂ united by the lever l. The automatic movement is obtained by the electromagnet R while a dashpot J prevents too sudden motion. Suppose the brushes to be in the position shown when the current would get too strong owing to lights being cut out. The electromagnet R getting stronger would raise A and reduce the current taken off until current came to normal. If, instead, some lamps were thrown in the current would become weak and the electromagnet R would become weak, drop A which would increase current and this will continue till current reaches normal.

The foregoing regulating gear is used on small dynamos and old style large ones. On the large new style dynamo a more delicate regulating gear is used, the current which operates it being shown at Fig. 19. Normally the electromagnet R is short circuited by the wire r and only acts when this circuit is broken. At some point in the main circuit is a wall controller or controller magnet shown in Fig. 19, at ST, consisting of two electro magnet, Their yoke supported by a spring and the yoke operating the contact lever S. If the current becomes too strong the controller magnet circuit is broken and all the current of the main circuit goes through the electromagnet R which by its sudden increase of strength quickly raises A and thus alters the brushes. This only exists for a moment until the yoke of the controller magets fall because of their decrease of magnets strength, when current again flows through wire r because when yoke drops contact is made. This decreases the strength of electro magnet R thus dropping A and increasing current. Hence S will again raise and break contact and R again rais A. This is continually repeated.