A clapper loader, or second assistant camera (2nd AC) is part of a film crew

Film crew

Television crew positions are derived from those of film crew positions.A film crew is a group of people hired by a production company for the purpose of producing a film or motion picture. Crew are distinguished from cast, the Actors who appear in front of the camera or provide voices for...

, whose main functions are that of loading the raw film stock

Film stock

Film stock is photographic film on which filmmaking of motion pictures are shot and reproduced. The equivalent in television production is video tape.-1889–1899:...

 into camera magazine

Camera magazine

A camera magazine is a light-tight chamber or pair of chambers designed to hold and move motion picture film stock before and after it has been exposed in the camera...

s, operating the clapperboard

Clapperboard

A clapperboard is a device used in filmmaking and video production to assist in the synchronizing of picture and sound, and to designate and mark particular scenes and takes recorded during a production...

 (slate) at the beginning of each take, marking the actors as necessary, and maintaining all records and paperwork for the camera department. The name "clapper loader" tends to be used in the United Kingdom and Commonwealth, while "second assistant camera" tends to be favoured in the United States, but the job is essentially the same whichever title is used. The specific responsibilities and division of labor within the department will almost always vary depending on the circumstances of the shoot.

Responsibility

The clapper loader typically has a very high responsibility level on any given set, because they are more or less the only person on set who directly and physically oversees the state of the undeveloped negative

Negative (photography)

In photography, a negative may refer to three different things, although they are all related.-A negative:Film for 35 mm cameras comes in long narrow strips of chemical-coated plastic or cellulose acetate. As each image is captured by the camera onto the film strip, the film strip advances so that...

. The loader is the only person who actually handles the negative between the manufacturer and the laboratory, and thus can easily render an entire day's work useless if the film is handled improperly. Additionally, the loader usually is in charge of keeping all records with regard to the film stock from when it is received until when it is sent out to the lab; if this information is miscommunicated or missing, this too can destroy an expensive shoot. Furthermore, the loader usually has much more to do in addition to these tasks. Noted Director of Photography Oliver Stapleton

Oliver Stapleton

Oliver Stapleton, B.S.C. is an English cinematographer.-Life and career:He graduated from the University of Cape Town in 1970 with a degree in psychology and from the National Film and Television School UK while working in South Africa and England.One of his first efforts was the student film...

 has written on his website:

Duties

A full description of the job duties includes the following (although different shoots may often not always require all of these):

• generally assisting the rest of the camera crew (focus puller
  Focus puller
  A focus puller, or 1st assistant cameraman, is a member of a film crew’s camera department whose primary responsibility is to maintain image sharpness on whatever subject or action is being filmed....
  
  , camera operator
  Camera operator
  A camera operator or cameraman is a professional operator of a film or video camera. In filmmaking, the leading cameraman is usually called a cinematographer, while a cameraman in a video production may be known as a television camera operator, video camera operator, or videographer, depending on...
  
  , director of photography)
• utilizing the camera trainee, film loader, and/or camera runner if one has been brought onto the production
• keeping inventory of all equipment, film, and expendables
• requesting film stock as needed
• securing the equipment
• checking loading materials and spaces to prevent light leaks
• cleaning and keeping clean the magazines and the loading environment
• organizing and cleaning the equipment space
• maintaining and cleaning the equipment
• loading and unloading of film stock from and to the magazines
• labelling of equipment, boxes, magazines, and storage spaces
• marking actors and props (leaving a marker of their positions as the scene is blocked
  Blocking (stage)
  Blocking is a theatre term which refers to the precise movement and positioning of actors on a stage in order to facilitate the performance of a play, ballet, film or opera. The term derives from the practice of 19th century theatre directors such as Sir W. S...
  
  for the purpose of measuring distance from the camera so that its focus can be adjusted throughout the scene)
• marking and operating the clapperboard properly
• keeping meticulous and accurate camera notes
• writing negative report sheets in detail
• interfacing with continuity in order to note which takes to print
• charging of batteries for camera and accessories
• preparation of film to be sent to the lab
• keeping records of time, per diems, and expenses for the entire camera crew
• liaising regularly with production, rental houses, editing, laboratories, and unions
• recordkeeping of all camera-related paperwork, including negative reports, daily stock reports, film inventory reports, lab orders, rental contracts, and expendable orders
• ensuring that all instructions from the director of photography are passed along properly to labs and post houses
• relaying reports from the lab about the rushes to the director of photography

On top of all of that, it is expected that a decent assistant will be able to anticipate a good part of those demands before they become demands.