Kellogg Receiver.  In Fig. 51 is shown a bi-polar receiver with internal or concealed binding posts.  This particular receiver is typical of a large number of similar kinds and is manufactured by the Kellogg Switchboard and Supply Company.  Two straight permanently magnetized bar magnets 1-1 are clamped together at their opposite ends so as to form a horseshoe magnet.  At the end opposite the diaphragm these bars clamp between them a cylindrical piece of iron 2, so as to complete the magnetic circuit at the end.  At the end nearest the diaphragm they clamp between them the ends of the soft iron pole pieces 3-3, and also a block of composite metal 4 having a large circular flange 4’ which serves as a means for supporting the magnet structure within the shell.  The screws by means of which the disk 4’ is clamped to the shouldered seat in the shell do not enter the shell directly, but rather enter screw-threaded brass blocks which are moulded into the structure of the shell.  It is seen from this construction that the diaphragm and the pole pieces and the magnet structure itself are all rigidly secured together through the medium of the shell at a point as close as possible to the diaphragm.

Between the magnets 1-1 there is clamped an insulating block 5, to which are fastened the terminal plates 6, one on each side of the receiver.  These terminal plates are thoroughly insulated from the magnets themselves and from all other metallic parts by means of sheets of fiber, as indicated by the heavy black lines.  On these plates 6 are carried the binding posts for the receiver cord terminals.  A long tongue extends from each of the plates 6 through a hole in the disk 4’, into the coil chamber of the receiver, at which point the terminal of the magnet winding is secured to it.  This tongue is insulated from the disk 4’, where it passes through it, by means of insulating bushing, as shown.  The other terminal of the magnet coils is brought out to the other plate 6 by means of a similar tongue on the other side.

In order that the receiver terminals proper may not be subjected to any strain in case the receiver is dropped and its weight caught on the receiver cord, a strain loop is formed as a continuation of the braided covering of the receiver cord, and this is tied to the permanent magnet structure, as shown.  By making this strain loop short, it is obvious that whatever pull the cord receives will not be taken by the cord conductors leading to the binding posts or by the binding posts or the cord terminals themselves.

A number of other manufacturers have gone even a step further than this in securing permanency of adjustment between the receiver diaphragm and pole pieces.  They have done this by not depending at all on the hard rubber shell as a part of the structure, but by enclosing the magnet coil in a cup of metal upon which the diaphragm is mounted, so that the permanency of relation between the diaphragm and the pole pieces is dependent only upon the metallic structure and not at all upon the less durable shell.