Abstract: A generator (10) comprising a series of spaced annular stators (15) sandwiched between a series of rotors (11), the rotors (11) each being separated by annular collars (16), the annular collars (16) defining a central cavity; at least one cooling gas source for supplying gas to the central cavity; vents (23,24,25,26,27,28,31,32,33,34,35) through the annular collars (16) for providing a means of egress for the cooling gas (20) from the central cavity radially outwards over the rotors (11) and the annular stators (15).