the outlet of the volute 18 and the inlet 30 an impeller 32 is driven by the motor shaft 16 of the electric motor 4 at the opposite end thereof to the end at which impeller 14 is driven the gas is driven by impeller 32 into a volute 34 from the gas is expelled at a higher pressure than the gas entering compressor 3 from inlet 30 although not illustrated a conventional outlet in the volute 34 is provided for discharging the gas at such higher pressure a back disk seal 36 is provided to prevent the escape of high pressure gas from the back face of the impeller 32 the back disk seal 36 is supported by a seal holder 38 a cavity 42 is formed behind the impeller 32 that lies between the back disk seal 36 and the motor shaft 16 a shaft seal 43 that provides a seal about the motor shaft 16 and therefore also seals cavity 42 shaft seal 43 is held in place by a seal holder 38 the shroud 12 is connected to the volute 18 which is in turn connected to one end of a cylindrical motor casing 44 the shroud 28 is connected to the volute 34 which is in turn connected to a motor cap 45 motor cap 45 is connected to the opposite end of cylindrical casing 44 the electric motor 4 has a rotor.