17.5 mm film was a film gauge for as many of eight types of motion picture film stock, created by splitting unperforated 35 mm film.
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History
In additional to original pioneering experiments, 17.5 mm film was used during World War II to use existing 35 mm stock more economically. Afterward, this format continued to be used primarily in developing countries or for projects, where usage of regular 35 mm film would have been too expensive.
Birtac
A british-american photographer and inventor Birt Acres slitted 35 mm film in half for his Birtac camera in 1898. The camera was using the film with two holes on the side. Historically this is considered the first device which used 17.5 film.
Biokam
Alfred Wrench and Alfred Darling created second 17.5 mm format film in 1898 in London. The difference between their and Acres' film was that their film had central perforation. The Biocam camera they used was as camera and projector and it was produced by the Warwick Trading Company.