An electrical machine comprising: a stator comprising: a core having a first end and a second end; one or more windings carried by the core and having: one or more first portions protruding from the first end; and one or more second portions protruding from the second end; a rotor mounted for rotation relative to the stator about an axis (500, 502); one or more chambers (70, 72) surrounding the one or more first portions and the one or more second portions; ferromagnetic particles in the one or more chambers; and means (80, 82, 104, 106) for varying a concentration of the ferromagnetic particles in the one or more chambers, characterised in that: the ferromagnetic particles are in a carrier liquid in the one or more chambers, wherein the carrier liquid comprises an oil, and in that the means comprises: a flowpath extending from an outlet (92-1, 92-2) of the one or more chambers and returning to an inlet (112-1, 112-2) of the one or more chambers; at least one pump along the flowpath; a separator along the flowpath; a first branch of the flowpath between the separator and the inlet; a second branch of the flowpath between the separator and the inlet, the second branch positioned to preferentially draw said ferromagnetic particles from the separator relative to the first branch; and one or more controlled valves (104, 106) positioned for apportioning flow between the first branch and the second branch. a stator comprising: a core having a first end and a second end; one or more windings carried by the core and having: one or more first portions protruding from the first end; and one or more second portions protruding from the second end; a core having a first end and a second end; one or more windings carried by the core and having: one or more first portions protruding from the first end; and one or more second portions protruding from the second end; one or more first portions protruding from the first end; and one or more second portions protruding from the second end; a rotor mounted for rotation relative to the stator about an axis (500, 502); one or more chambers (70, 72) surrounding the one or more first portions and the one or more second portions; ferromagnetic particles in the one or more chambers; and means (80, 82, 104, 106) for varying a concentration of the ferromagnetic particles in the one or more chambers, characterised in that: the ferromagnetic particles are in a carrier liquid in the one or more chambers, wherein the carrier liquid comprises an oil, and in that the means comprises: a flowpath extending from an outlet (92-1, 92-2) of the one or more chambers and returning to an inlet (112-1, 112-2) of the one or more chambers; at least one pump along the flowpath; a separator along the flowpath; a first branch of the flowpath between the separator and the inlet; a second branch of the flowpath between the separator and the inlet, the second branch positioned to preferentially draw said ferromagnetic particles from the separator relative to the first branch; and one or more controlled valves (104, 106) positioned for apportioning flow between the first branch and the second branch. the ferromagnetic particles are in a carrier liquid in the one or more chambers, wherein the carrier liquid comprises an oil, and in that the means comprises: a flowpath extending from an outlet (92-1, 92-2) of the one or more chambers and returning to an inlet (112-1, 112-2) of the one or more chambers; at least one pump along the flowpath; a separator along the flowpath; a first branch of the flowpath between the separator and the inlet; a second branch of the flowpath between the separator and the inlet, the second branch positioned to preferentially draw said ferromagnetic particles from the separator relative to the first branch; and one or more controlled valves (104, 106) positioned for apportioning flow between the first branch and the second branch. a flowpath extending from an outlet (92-1, 92-2) of the one or more chambers and returning to an inlet (112-1, 112-2) of the one or more chambers; at least one pump along the flowpath; a separator along the flowpath; a first branch of the flowpath between the separator and the inlet; a second branch of the flowpath between the separator and the inlet, the second branch positioned to preferentially draw said ferromagnetic particles from the separator relative to the first branch; and one or more controlled valves (104, 106) positioned for apportioning flow between the first branch and the second branch.