Abstract :
One of the troubles experienced in the collection of current is that of ¿copper picking¿. A description is given of an experimental investigation of this phenomenon, and this is followed by a critical survey of existing data, leading to an explanation to account for this deposition of metal, and an investigation into the mechanism of the passage of current from the slip-ring or commutator to the brush. The variation of contact voltage-drop with speed, brush pressure and current is shown. Peripheral speeds higher than previously have been used and an investigation has been made at very low current densities. In order to show the variation of contact voltage-drop, oscillograms have been taken which show its fluctuating nature. These oscillograms show the effect of increased speed and brush pressure, and the change in nature of the contact as the latter is reduced. Intermittent contact is shown to account for rising voltage-drop with speed, but a speed is attained at which an air film is drawn between the brush and the ring.
