In 1920, Bell & Howell introduced the Cinemotor, a revolutionary electric motor drive for their model 2709 motion picture camera. Research suggests that Bell & Howell was the first to introduce an electric camera motor that incorporated a centrifugal governor and a rheostat in order to achieve constant speeds at variable frame rates. For its time, this was quite an innovation, as it freed the camera operator from hand cranking the camera and made various in-camera effects easier to achieve. Using the hand dissolve attachment, along with the variable speed functionality of the motor, the camera operator could control the shutter angle and film speed simultaneously. This innovation influenced the way motion pictures were made moving forward. Here is an example of one of the Cinemotors we have in our inventory. A wonderful example of how the art of filmmaking was refined over the decades.
Diagram from the patent application for the Bell & Howell Cinemotor. 
Bell & Howell Cinemotor 
Rear view of the Bell & Howell Cinemotor. 
A view of the tag on the side of a Bell & Howell Cinemotor.