American cinematographer (Jan-Oct 1925)

July, 1925 AMERICAN CIN (a) In order to determine the effect of humidification in atmospheres of increasing relative humidity on the propensity of gelatine and film base to generate static electricity, strips of motion picture positive film and sheets of gelatine were exposed to atmospheres of different humidities by placing in humidors containing sulphuric acid of varying concentrations (representing atmospheres of known relative humidity) and stored for 12 hours at temperatures of 50"F. and 110"F., respectively. The strips were then rubbed vigorously with a piece of velvet (the positive film was rubbed on the base side) and tested for electrification by means of an electroscope. The results obtained were as follows: Relative Electrification Humidity Material 50 cleg. F. HO deg. F. 54% Gelat ine slight si ight M. P. Positive Film strong strong 74% Gelatine slight slight M. P. Positive Film slight slight 82% Gelatine nil nil M. P. Positive Film slight very slight 88% Gelatine nil nil M. P. Positive Film very slight nil 02% Gelatine nil nil M. P. Positive Film nil nil From these tests it is seen that gelatine ceases to generate an appreciable amount of static electricity when exposed to an atmosphere of about 80 % relative humidity for twelve hours, at 50°F. Although tests were not made with sheets of emulsion stripped from the base, comparative tests made by rubbing gelatine sheets and the emulsion side of motion picture film exposed to the same atmosphere, showed that positive and negative motion picture emulsions have less tendency to generate static electricity than plain gelatine. The above tests also show that with motion picture negative film the air must have a relative humidity of about 90% at 50°F. and about 85% at 110"F. if it is to entirely prevent the generation of static electricity when the film is exposed to it for a few hours. Since with air at any constant relative humidity the quantity of water which it contains increases with rise of temperature, film in equilibrium with such air contains a greater quantity of water at higher temperatures. Since the propensity of film to give static markings runs parallel with the absolute quantity of moisture which it contains, it would be expected that at a given relative humidity the propensity of film to give static would decrease with rise of temperature, as was shown by the above experiments. (b) A dry emulsion or a dry film base absorbs moisture comparatively slowly. Bone dry motion picture film must be humidified for more than 24 hours in an atmosphere at 80% to 90% relative humidity before it absorbs all the moisture it will hold under these conditions. Hence, the condition of the air has very little effect unless the film is exposed to it for a sufficient length of time. Thus, dry motion pic MATOGRAPHER Nineteen PYREX Spotlamp Condensers ARE GUARANTEED AGAINST BREAKAGE 6-in. — 8-in. — 12-in. diameters Used by: Famous Players-Lasky, United. Metro-Gold wyn-Mayer, First National studios. Sole Distributors: THE LUXALBA COMPANY 111 West 42nd St., New York Just one production with us will make you our friend for life. Roy Davidge Film Laboratories 6701 Santa Monica Blvd. HOlly 1944 FOR RENT Two Bell and Howell Cameras, 40, 50, 75 mm. lenses, Thalhammer iris. Jean Trebaol. Jr., 7042 Stilson Street, Palms, Calif. Telephone EMpire 8954. SUBSCRIBE FOR THE American Cinemato qrapher