Such a receiver is shown in cross-section in Fig. 57, and completely assembled with its head band in Fig. 58.  Referring to Fig. 57 the shell 1 of the receiver is of aluminum and the magnets are formed of steel rings 2, cross-magnetized so as to present a north pole on one side of the ring and a south pole on the other.  The two L-shaped pole pieces 3 are secured by screws to the poles of these ring magnets, and these pole pieces carry the magnet coils, as is clearly indicated.  These poles are presented to a soft iron diaphragm in exactly the same way as in the larger hand receivers, the diaphragm being clamped in place by a hard rubber ear piece, as shown.  The head bands are frequently of steel covered with leather.  They have assumed numerous forms, but the general form shown in Fig. 58 is the one commonly adopted.

[Illustration:  Fig. 58.  Operator’s Receiver and Cord]

[Illustration:  Fig. 59.  Receiver Symbols]

Conventional Symbols.  The usual diagrammatic symbols for hand and head receivers are shown in Fig. 59.  They are self-explanatory.  The symbol at the left in this figure, showing the general outline of the receiver, is the one most commonly used where any sort of a receiver is to be indicated in a circuit diagram, but where it becomes desirable to indicate in the diagram the actual connections with the coil or coils of the receiver, the symbol shown at the right is to be preferred, and obviously it may be modified as to number of windings and form of core as desired.

CHAPTER VII

PRIMARY CELLS

Galvani, an Italian physician, discovered, in 1786, that a current of electricity could be produced by chemical action.  In 1800, Volta, a physicist, also an Italian, threw further light on Galvani’s discovery and produced what we know as the voltaic, or galvanic, cell.  In honor of these two discoverers we have the words volt, galvanic, and the various words and terms derived therefrom.

Simple Voltaic Cell.  A very simple voltaic cell may be made by placing two plates, one of copper and one of zinc, in a glass vessel partly filled with dilute sulphuric acid, as shown in Fig. 60.  When the two plates are not connected by a wire or other conductor, experiment shows that the copper plate bears a positive charge with respect to the zinc plate, and the zinc plate bears a negative charge with respect to the copper.  When the two plates are connected by a wire, a current flows from the copper to the zinc plate through the metallic path of the wire, just as is to be expected when any conductor of relatively high electrical potential is joined to one of relatively low electrical potential.  Ordinarily, when one charged body is connected to another of different potential, the resulting current is of but momentary duration, due to the redistribution of the charges and consequent equalization of potential.  In the case of the simple cell, however, the current is continuous, showing that some action is maintaining the charges on the two plates and therefore maintaining the difference of potential between them.  The energy of this current is derived from the chemical action of the acid on the zinc.  The cell is in reality a sort of a zinc-burning furnace.