The coils S, and S2 constitute part of a secondary circuit that includes a three-electrode vacuum tube, transformer, condensers, direct current motor and battery. The coils S,' and 82' constitute part of another secondary circuit. So long as the primary coil P is equally distant from the four secondary coils, zero electromotive force is induced in each secondary circuit and neither motor starts. This is the condition when the camera is vertical.
If the end A of the frame tilts upward, the system of four secondary coils becomes inclined to the horizontal plane of the coil P, Fig. 84a, coil P now is nearer S2 than to S, and it is equally distant from S,' and S2'. An electromotive force is being induced in S,S2 whereas zero electromotive force is being induced in Si S2'. The motor in the S,S2 circuit starts and tilts the frame till the plane of the secondary coils is parallel to the plane of the primary coil P, that is, till the camera axis becomes vertical.
In the same manner, if the frame becomes tilted about an axis AA', the distance between the primary coil and the two secondary coils Si and S2' becomes unequal. The motor in the Si S2' circuit starts and tilts the frame back till the camera axis is again vertical. The currents in the primary and secondary coils are so minute that these actions produce an inappreciable force on the gyro and therefore no precession.
64. Control of the Line of Sight of a Camera. - The camera, instead of being held vertical over the ground to be photographed, may be mounted rigidly along the fore-and-aft axis of the airplane and a vertical beam of light from the ground reflected into the lens system. This can be done by either a plane mirror or a totally refleetino nrism nleeed in front of the lens system. In order