attached by solder to the face of a brass disk having a rearwardly projecting screw-threaded shank, which serves to hold it and the rear electrode in place in the bottom of a heavy brass cup 4.  The front electrode is mounted on the rear face of a stud.  Clamped against the head of this stud, by a screw-threaded clamping ring 7, is a mica washer, or disk 6.  The center portion of this mica washer is therefore rigid with respect to the front electrode and partakes of its movements.  The outer edge of this mica washer is similarly clamped against the front edge of the cup 4, a screw-threaded ring 9 serving to hold the edge of the mica rigidly against the front of the cup.  The outer edge of this washer is, therefore, rigid with respect to the rear electrode, which is fixed.  Whatever relative movement there is between the two electrodes must, therefore, be permitted by the flexing of the mica washer.  This mica washer not only serves to maintain the electrodes in their normal relative positions, but also serves to close the chamber which contains the electrodes, and, therefore, to prevent the granular carbon, with which the space between the electrodes is filled, from falling out.

[Illustration:  Fig. 40.  White Solid-Back Transmitter]

The cup 4, containing the electrode chamber, is rigidly fastened with respect to the body of the transmitter by a rearwardly projecting shank held in a bridge piece 8 which is secured at its ends to the front block.  The needed rigidity of the rear electrode is thus obtained and this is probably the reason for calling the instrument the solid-back.  The front electrode, on the other hand, is fastened to the center of the diaphragm by means of a shank on the stud, which passes through a hole in the diaphragm and is clamped thereto by two small nuts.  Against the rear face of the diaphragm of this transmitter there rest two damping springs.  These are not shown in Fig. 40 but are in Fig. 41.  They are secured at one end to the rear flange of the front casting 1, and bear with their other or free ends against the rear face of the diaphragm.  The damping springs are prevented from coming into actual contact with the diaphragm by small insulating pads.  The purpose of the damping springs is to reduce the sensitiveness of the diaphragm to extraneous sounds.  As a result, the White transmitter does not pick up all of the sounds in its vicinity as readily as do the more sensitive transmitters, and thus the transmission is not interfered with by extraneous noises.  On the other hand, the provision of these heavy damping springs makes it necessary that this transmitter shall be spoken into directly by the user.

[Illustration:  Fig. 41.  White Solid-Back Transmitter]