The invention relates to a disc commutator and a drum commutator for an electric motor having at least two commutator plates, each having a sliding-surface segment for contacting a carbon brush, said sliding-surface segments being electrically separated from one another. In the disc commutator, the electrical separation extends between the sliding-surface segments towards a commutator diameter inclined at an angle In the drum commutator, the electrical separation of the sliding-surface segments is inclined in the longitudinal direction of the drum commutator and forms an angle with a projection of the rotational axis of the drum commutator on the casing surface of the drum commutator.

A laminated carbon brush for a liquid pump motor slides in a liquid fuel on a disk-like commutator. The laminated carbon brush includes two layers of a lower resistivity layer and a higher resistivity layer. In both the lower resistivity layer and the higher resistivity layer, circular directional resistivities of the brush along a circular direction in the rotation of the commutator are higher than non-circular directional resistivities of the brush along a radial direction of the commutator and a perpendicular direction to the sliding surface of the commutator. A non-circular directional resistivity of the higher resistivity layer are higher than or equal to 90,000.Math.cm. No spark discharges occur if the fuel pump motor is operated to output a high power.

A power connection device is provided for transmitting electric power from an external power source to a working machine). The working machine has an undercarriage and a superstructure, and the undercarriage and the superstructure are pivotable relative to each other about a substantially vertical pivot axis. The power connection device includes a first power conductor member which is adapted to be pivotally arranged relative to the undercarriage and the superstructure about the substantially vertical pivot axis.

A power connection device is provided for transmitting electric power from an external power source to a working machine). The working machine has an undercarriage and a superstructure, and the undercarriage and the superstructure are pivotable relative to each other about a substantially vertical pivot axis. The power connection device includes a first power conductor member which is adapted to be pivotally arranged relative to the undercarriage and the superstructure about the substantially vertical pivot axis.

An electrode connection structure includes a first electrode unit that includes first electrodes formed concentrically, and a second electrode unit that includes a second electrode formed into a needle shape axially movable forward and rearward and is electrically connectable to the first electrode unit, and in the electrode connection structure, the second electrode unit includes a plurality of the second electrodes that is arranged in a circumferential direction and arranged at different radial positions, and the plurality of the second electrodes is electrically connected to the first electrodes arranged at different radial positions, respectively.

An electrode connection structure includes a first electrode unit that includes first electrodes formed concentrically, and a second electrode unit that includes a second electrode formed into a needle shape axially movable forward and rearward and is electrically connectable to the first electrode unit, and in the electrode connection structure, the second electrode unit includes a plurality of the second electrodes that is arranged in a circumferential direction and arranged at different radial positions, and the plurality of the second electrodes is electrically connected to the first electrodes arranged at different radial positions, respectively.