Abstract: An electrical machine, such as an electric motor, generator or torquer-generator, includes a relatively rotatable cylindrical armature and magnetic pole structure separated by an annular gap into which a surrounding fluid may flow during operation of the machine. In order to prevent the build-up of deposited solid material in the gap between the components, one of the components is provided at its surface with a narrow elongate projection which extends helically along and around the surface. The height of the projection is slightly less than the gap between the components so that it remains out of contact with the surface of the other component so as not to frictionally engage it and interfere with relative rotation between the components.