If the threading has been accomplished properly the interior of the camera should be as shown in the illustration facing p. 42. Here both film boxes are shown open, though of course the upper one is kept closed during the threading process. The most important part of this operation is the formation of ample loops both above and below the gate. Once formed they remain constant, because of the action of the teeth on the sprocket; for the movement of the ribbon over the sprocket is equal to that produced by the claws through the gate. At first sight the necessity of these loops may not be apparent, but when it is remembered that the film is moved through the gate intermittently, sixteen times per second, with a vicious jerk, it will be seen that if there were no loop, and the film were drawn directly from the dark box, a great strain would be imposed upon it, and probably it would break. But by providing the loop an elastic feed is secured, and the film is jerked into position before the lens with the minimum of vibration and without any risk of tearing or displacement.
Threading completed the lower dark box is closed and secured, together with the side of the camera, so that the whole of the interior is light-proof. The camera should not be re-opened after photographing has commenced until the whole of the film in the loaded box is exhausted, or, if the supply is not used, until the dark-room is regained, unless the waste of a foot or two of film is a secondary consideration. Should it become necessary to open the camera in the field, the handle should be given two or three turns to make sure that the last picture taken is wound into the exposed film box and is thus secure from the light. Opening the side of the camera obviously ruins the whole of the unexposed film threaded through the mechanism, so that when picture taking is resumed the handle must be given a few more turns to make sure that the whole of the light-ruined film has passed through the gate. Seeing that one turn of the handle represents eight exposures, equivalent to 6 inches of film, it is a simple matter to estimate how many turns of the handle are necessary to clear the gate of spoiled film.
Most cameras are provided with a film indicator enabling the operator to tell at a glance how many feet of film have been exposed. In this case, care must be taken to see that the indicator is returned to zero when the mechanism is threaded-up.