CHAPTER XXXVII.

Anti-Sparking Dynamo Brush and Commutator.

.In direct current dynamos of great electromotive force—such,for instance, as those used for arc lighting—when one commuta-tor bar or plate comes out of contact with the collecting-brush aspark is apt to appear on the commutator. This spark may bedue to the break of the complete circuit, or to a shunt of lowresistance formed by the brush between two or more commuta-tor-bars. In the first case the spark is more apparent, as there isat the moment when the circuit is broken a discharge of themagnets through the field helices, producing a great spark, orflash which causes an unsteady current, rapid wear of the com-mutator bars and brushes, and waste of power. The sparkingmay be reduced by various devices, such as providing a path forthe current at the moment when the commutator segment or barleaves the brush, by short-circuiting the field-helices, by increas-ing the number of the commutator-bars, or by other similarmeans ; but all these devices are expensive or not fully available,and seldom attain the object desired.

To prevent this sparking in a simple manner, Mr. Tesla someyears ago employed with the commutator-bars and interveninginsulating material, mica, asbestos paper or other insulating andincombustible material, arranged to bear on the surface of thecommutator, near to and behind the brush.

In the drawings, Fig. 244 is a section of a commutator withan asbestos insulating device ; and Fig. 245 is a similar view, re-presenting two plates of mica upon the back of the brush.

In Fig. 244, c represents the commutator and interveninginsulating material; b b, the brushes, d d are sheets of asbestospaper or other suitable non-conducting material, f f are springs,the pressure of which may be adjusted by means of the screws9 9-

In Fig. 245 a simple arrangement is shown with two plates ofmica or other material. It will be seen that whenever one com-

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