* Canister (or cylinder) designs have the motor and bag in a separate canister unit (usually mounted on wheels) connected to the vacuum head by a flexible hose. Although upright units have been tested as more effective (mainly because of the beaters), the lighter, more manoeuverable heads of canister models are popular. Some upmarket canister models have "power heads", which contain the same sort of mechanical beaters as in upright units, although such beaters are driven by a separate electric motor.