86. Description of the UX-867 Photo-Electric Cell.-The UX-867 cell is illustrated in Figure 58, and
consists of a glass bulb connected by a glass neck to a standard radio tube base. The inner surface of
the bulb is coated with metallic silver except for a small window on one side. This silver coating is in
turn coated with a thin layer of caesium (the active material). The purpose of the silver coating is to
make a good electrical connection to the coating of caesium. An external connection to the silver
coating is made by means of a short lead from the inner surface of the glass to a metal tip at the outside
of the glass. There are two wires at the center of the bulb which are connected together in the base and
connected to one prong. These wires form the anode of the cell. This cell is not used in many of the
RCA Photophone equipments at present, the UX-868 cell being used almost exclusively.
87. Description of the UX-868 Photo-Electric Cell.-Figure 59 is an illustration of the UX-868 cell. It
consists of a curved plate or cathode coated with caesium, and a wire anode mounted parallel to the
long axis of the plate. This cell, in spite of its small size, is very sensitive and highly efficient, as photoelectric cells go. Its small size makes it admirably suited for use in sound on film reproducing equipment.
The large output of these and the UX-867 tubes make it unnecessary to mount a vacuum tube amplifier
close to the cell and eliminates the necessity of the unsatisfactory practice of mounting vacuum tubes
on the projector.
(B) RCA PHOTOPHONE SOUND HEADS
88. Description of the PS-1 Sound Heads.-The Type PS-1 sound heads are used with PG-1, 2, 3, 4, 6, 7
and 8 equipments. The type PS-1 sound head consists of a housing containing an exciter lamp
assembly; an optical system, for focusing the light on the film; a sound gate for guiding the film past
the beam of light; a viscous damping device to insure an absolutely constant speed of the film; and a
photocell and its transformer for translating the light variations into electrical variations. (See Figure 60.)
89. Exciter Lamp Assembly of PS-1 Sound Heads.-Three exciter lamps are mounted on a turret in this
sound head. Only one of the lamps is used at a time, but any one of them can be lined up quickly in
front of the optical system by merely rotating the turret by means of a handle provided for the purpose.
The reason for having the three lamps is that, in case the one in use should burn out, a new lamp can be
quickly rotated into place. These lamps should be adjusted for proper setting when they are mounted,
to prevent delay when it becomes necessary to change lamps. The lamps are adjusted by raising or
lowering them in their sockets. They should be adjusted to give a round circle of white light on a card
placed between the sound gate and photo-cell housing. If the adjustment is not right the circle will not
be complete. In such cases the lamp should be raised or lowered until a full circle of light is obtained.
The lamp should then be clamped in place with the fingers by
means of the knurled nut. Do not use a pair of pliers, as the exciter lamp base may become cracked.
When the lamp is once properly focused it need not be touched again.