Clap boy

A clap boy or clapper-boy is a defunct movie industry profession. This person loaded film into and out of film magazines in cameras in early film making.

The most up-to-date job title for someone doing this is second assistant camera (2AC). The job is an important one in the scheme of things as the 2AC's function is to assist the first assistant camera at all times as well as operate the zoom action for the camera operator during shots. For all that, 2ACs are the lowest person on the camera-crew totem.