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72 Transactions of S.M.P.E., August 1925
and since the rate of evaporation depends on the dryness or relative humidity of the air, the difference in reading between the wet and dry thermometers is a measure of the relative humidity of the air. It is important when using a hygrometer to place it in such a position that a representative sample of the air circulates over it. By reference to tables supplied with the instrument the relative humidity is obtained. Some hygrometers rely on the expansion and contraction of a strand of horsehair in dry and moist air but these are not always reliable.
If motion picture film is placed in an atmosphere at any relative humidity there is an exchange of moisture either from the film to the air or vice versa until equilibrium is reached. That is, dry film in a moist atmosphere absorbs water while moist film in a dry atmosphere loses water.
The transfer of moisture either from the air to the film or vice versa requires time and takes place comparatively slowly.
Since the tendency of film to give static markings depends on its conductivity which in turn depends on the absolute quantity of water which it contains, the effect of moist air in affecting the propensity of film to give static depends on
a. The relative humidity and temperature of the air.
b. The time of exposure of the film to the air.
(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 Humidity
54% 74% 82% 88% 92%
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.
Electrification
y Material 50° F.
110" F.
Gelatine slight
slight
M. P. Positive Film strong
strong
Gelatine slight
slight
M. P. Positive Film sHght
shght
Gelatine nil.
nil.
M. P. Positive Film slight
very slight
Gelatine nil.
nil.
M. P. Positive Film very slight nil.
Gelatine nil.
nil.
M. P. Positive Film nil.
nil.