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The Construction Of Early Telephone Receivers

Feb 29, 2008
To construct a receiver capable of reproducing speech is a very simple matter. In fact, nearly any electromagnet, with a comparatively light iron armature, such as is commonly used in electric bells and telegraph instruments, may be made to reproduce, with more or less distinctness, sounds uttered in the vicinity of a transmitting apparatus with which it is in circuit.

It has proved more difficult, however, to construct a receiving instrument which will reproduce speech well, and at the same time be practically successful in everyday use.

The bar-magnet with a thin iron diaphragm in close proximity to one of its poles, used in the early experiments in telephony, has until recently been very generally adhered to throughout this country.

The instrument has been made much more sensitive than were the early forms, but this result has been accomplished by better mechanical and electrical designs, and the use of better materials, and not by any departure from the original principles of its action.

Aside from actual talking efficiency, many considerations of a purely mechanical nature enter into the design of a good telephone receiver. It should be durable and capable of withstanding the rough usage to which it will necessarily be subjected by careless or ignorant users.

It should be of such construction that its adjustment will not be changed by mechanical shocks or by changes in temperature.

Failure to provide against this latter effect is one of the chief sources of trouble in telephone work. It should be of such external configuration as to enable it to be conveniently placed to the ear. The chamber in which the diaphragm vibrates should be small and of such shape as not to muffle the sound.

The binding posts should be so securely fastened in as to prevent their becoming loose and twisting off the wires inside the receiver shell ; and the construction should be so simple as to render the replacing of any damaged part an easy matter.

By far the most popular early receivers used in America were of the single-pole type. This particular form proved efficient, and was largely used by the American Bell Telephone Company. Its chief merit lies in its simplicity.

It is essentially a compound bar-magnet, composed of two pairs of separately magnetized steel bars arranged with like poles together. Between the pairs of bars is clamped a soft-iron polepiece at one end, and a similarly shaped iron block at the other end. These parts are firmly bound together by the two screws.

On the end of the pole-piece is slipped a coil of wire. This coil is usually wound with two parallel silk-insulated copper wires, and has a total resistance of about 75 ohms.

The magnet is incased in a shell of hard rubber, composed of two pieces, which screw together and clamp between them the diaphragm of thin sheet iron. The piece is hollowed out to form a convenient earpiece. A tailpiece carrying two binding posts fits over the end of the case opposite the earpiece, and is held in place by a screw.

This screw engages a threaded hole in the block and serves not only to hold the tail-piece in place, but to bind the magnet securely to the shell. Soldered to the binding posts are heavy leading-in wires, which pass along the sides of the magnet and are soldered to the respective terminals of the fine wire forming the coil.

In some single-pole receivers the old style of magnet, consisting of a single cylindrical bar of steel, was used instead of the compound magnet formed of several separately magnetized bars, but with generally inferior results, owing to its weaker and less permanent magnetic field.

In bipolar receivers, which later came into general use, the object is to strengthen the field in which the diaphragm vibrates, by presenting both magnet poles to the diaphragm.

The length of the path of the lines of force through the air is thus greatly shortened, and the field of force is concentrated at the point where it will be most effective.

A popular form of bipolar receiver was produced by one of the large independent companies, and is typical of this design. The shell and ear-piece are of a material resembling hard rubber, and clamp between them the soft-iron diaphragm. The magnet consists of two pairs of separately magnetized steel bars, the separate bars in each pair being laid with like poles together, so that each pair forms in itself a compound bar-magnet.

These two compound bar-magnets are so laid together that the north pole of one is opposite the south pole of the other. The two pairs of bars are held apart at one end by the adjustment block made of the same material as the shell.

Over time, these designs gave way to more sophisticated designs, composing solid state elements and offering a more true to life reproduction of sound. Today, we owe a great debt to these early inventions, which make all modern telephones possible.
About the Author
Malcolm Blake is a telephone enthusiast who embraces the best of the golden age of telephones along with the modern era. He has written a number of articles about modern telephony, including advice on how to do a reverse number look up.
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