The entire working parts of this receiver are contained within the cup 5, the edge of which is flanged outwardly to afford a seat for the diaphragm.  The diaphragm is locked in place on the shell by a screw-threaded ring 6, as is clearly indicated.  A ring 7 of insulating material is seated within the enlarged portion of the barrel 1, and against this the flange of the cup 5 rests and is held in place by the cap 2 when it is screwed home.  The working parts of this receiver partially disassembled are shown in Fig. 56, which gives a clear idea of some of the features not clearly illustrated in Fig. 55.

It cannot be denied that one of the principal items of maintenance of subscribers’ station equipment has been due to the breakage of receiver shells.  The users frequently allow their receiver to fall and strike heavily against the wall or floor, thus not only subjecting the cords to great strain, but sometimes cracking or entirely breaking the receiver shell.  The innovation thus proposed by the Dean Company of making the entire receiver shell of steel is of great interest.  The shell, as will be seen, is entirely insulated from the circuit of the receiver so that no contact exists by which a user could receive a shock.  The shell is enameled inside and out with a heavy black insulating enamel baked on, and said to be of great durability.  How this enamel will wear remains to be seen.  The insulation of the interior portions of the receiver is further guarded by providing a lining of fiber within the shell at all points where it seems possible that a cross could occur between some of the working parts and the metal of the shell.  This type of receiver has not been on the market long enough to draw definite conclusions, based on experience in use, as to what its permanent performance will be.

Thus far in this chapter only those receivers which are commonly called hand receivers have been discussed.  These are the receivers that are ordinarily employed by the general public.

[Illustration:  Fig. 57.  Operator’s Receiver]

Operator’s Receiver.  At the central office in telephone exchanges the operators are provided with receivers in order that they may communicate with the subscribers or with other operators.  In order that they may have both of their hands free to set up and take down the connections and to perform all of the switching operations required, a special form of receiver is employed for this purpose, which is worn as a part of a head-gear and is commonly termed a head receiver.  These are necessarily of very light construction, in order not to be burdensome to the operators, and obviously they must be efficient.  They are ordinarily held in place at the ear by a metallic head band fitting over the head of the operator.

[Illustration:  GRANT AVENUE OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO, CAL.  A Type of Central-Office Buildings in Down-Town Districts of Large Cities.]