TransactionsPublished under the care of the General Secretary and the Treasurer . n. ARNOLD AND LA COUR: COMMUTATION. 80/ If the collector-ring is heated by means of a flame, the strengthof the current, at constant voltage, increases. Also the pressureof the carbons has a considerable influence on the voltage appears plainly from Fig. 7. Oiling and polishing the col-lector causes a slight increase in resistance, by which the produc-tion of sparks on the cornmutator can, under some circumstances,be avoided. In general, however, when using carbon brushes, thelubrication of the commuta
TransactionsPublished under the care of the General Secretary and the Treasurer . n. ARNOLD AND LA COUR: COMMUTATION. 80/ If the collector-ring is heated by means of a flame, the strengthof the current, at constant voltage, increases. Also the pressureof the carbons has a considerable influence on the voltage appears plainly from Fig. 7. Oiling and polishing the col-lector causes a slight increase in resistance, by which the produc-tion of sparks on the cornmutator can, under some circumstances,be avoided. In general, however, when using carbon brushes, thelubrication of the commutator is not advisable, since it thenreadily blackens. In Fig. 8 are given voltage curves for three kinds of carbonsmanufactured by the firm Le Carbone. These curves have beentaken from a commutator at 12 meters-per-second surface current was passed through one carbon brush to the com-mutator, and taken from it through the other. The voltage A-^is plotted as a function of the effective current-density ,9^^Curve / refers to hard carbons of Brand S, curve // to medium. 0 Z 4 6 8 10 12 14 16 18 8. Transition Voltages of Le Carbone of Brand Z and curve III to soft carbons of Brand the carbons were well started, the strongest current was sentthrough them, so that they were thoroughly warmed. Current andvoltage with different currents were then recorded in the quickestpossible succession. The carbons and the commutator had conse-quently no time to cool off, and the curves obtained refer to almostthe same temperature of contact surfaces. In Figs. 9 and 10 the voltage curve A^ for copper andBoudreaux brushes is plotted as a function of the current strength. The losses in the commutator are composed of the losses frommechanical friction and the losses through the transition of thecurrent. The losses from rubbing are known to be:Wr= Vk -Fb ff P watts. 808 ARNOLD AND LA COUR: COMMUTATION. Where g = pressure in Kg per cm^.p = coeffifuent of f
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