. Le chemin de fer de rue critique . t, le com-mutator propre et les brosses en bon état. La dynamo en question a déjà travaillé correctement et le circuit d'égalisation d'une résistance suffisamment faible à la régulation d'assureguood. Il serait maintenant conseillé de tester les bobines de champ de shunt, qui peuvent être facilement réalisées dans la lampe illustrée à la Fig. i. Laissez G, H, I, J, représenter les coilsà tester ; organisez une série de cinq contacts ou des postsA„ B2, C, DJ et EJ, et faites passer des fils fortement isolés de a, B, à Bo, C, à Co, D, à faire, et E, à E,,,,. Relevés du voltmètre entre A- et B^ wi

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. The street railway review . t, the com-mutator clean and brushes in good order. The dynamo inquestion has previously been working properly and theequalizing circuit of sufficiently low resistance to assuregood regulation. It would now be advisable to test theshunt field coils, which can be readily accomplished in themanner illustrated in Fig. i. Let G, H, I, J, represent coilsto be tested ; arrange a series of five contacts or binding postsA„ B2, C, , Dj and Ej, and run heavily insulated wires froma to A, , B, to Bo, C, to Co, D, to Do, and E, to E, , . Volt-meter readings between A-, and B^ will give e. m. f. of coilG ; B, , Cj coil II, Cj D, coil I and D, E^ coil J. Troublein any of the coils would be known by its relatively low e.m. f. and would account for the dynamo shirking its load. It is important that the commutator brushes be kept asnear to the neutral or non-sparking position, as the variationsin load will permit. It is necessary with nearly all dyna-mos to set the lead forward or backward when the load. FIG. I.—CONNECTIONS FOR TESTING COILS. makes a pronounced change either increasing or decreasing.Machines should be compounded so that the load can bekept equally divided between the different units. The leadsshould be the same, then if a sudden load should come on ora heavy short circuit should occur, the load would not bethrown on one machine because of an excessive forward leadweakening the fields of some, and a less lead of brushesleaving the other with a stronger magnetic field. This isdue to the distortion of the field, by the magnetizing effectof the current Howing in the armature. Short circuits in commutators are caused in most instancesby the pitting or burning away of the mica insulation be-tween the segments; carbon and copper dust get in andare burned into a hard mass of sufficiently low resistance toshort circuit the coil to which the segments arc connected, and produce an excessive How of current in the coil. Thiscauses the brushes to spit

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