Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

The only type of receiver of much service in wireless telegraphy is that known as a watch case or pony receiver. It is small and compact so that it may be attached to a headband and clamped against the ear.

Fig. 142. Types of Permanent Magnets.

The permanent magnets of a watch-case receiver are usually in the form of either a ring or a horseshoe as shown by Fig. 142. The first form has "consequent" poles and is considered somewhat superior to the horseshoe, since the lines of force are not so liable to pass across the pole pieces before they pass through the electromagnets and the diaphragm.

The ordinary low resistance telephone receiver is perfectly well suited to the telephone work for which it was designed and adjusted, and will give good service on a wireless receptor for short distances, but can be considerably improved by following the suggestions and instructions given below.

The principal objection to the ordinary receiver is that it does not contain enough turns of wire on its bobbins. This is easily remedied by carefully rewinding them with a very fine silk covered, pure copper magnet wire no larger than No. 40 B. S. gauge. This will increase the number of turns and also the resistance, but it must not be inferred that resistance is to be desired. This is a common impression of amateurs who do not understand the underlying principle, that the strength of an electromagnet varies directly as the number of turns of wire, multiplied by the amperes flowing through the magnet. When a telephone receiver is wound with a finer wire the resistance is increased, cutting down both the current and the strength of the magnet. But if pure copper wire is used, and the winding not carried beyond the point where the circumference of the outside layer becomes twice as great as the circumference of the first layer, the number of turns increases faster than the resistance and the magnet strength is considerably greater than before the receiver was rewound.

No. 40 B. S. gauge silk covered wire is often used for this purpose, but the best results are obtained with enameled covered wire of the same size. It is possible to wind almost three times as much of the enameled wire on a telephone bobbin as silk wire of the same size. The difference is due to the thickness of the insulation. An ordinary double pole watch-case receiver will have a resistance of 800-1,000 ohms when wound with silk covered wire and 1,500-1,800 ohms when wound with enameled wire.

To rewind a telephone receiver, first unscrew the cap and remove the diaphragm, then remove the bobbins by unloosening the screws with the aid of a screw driver. Unwind the old wire and examine the empty bobbins to see that wherever the wire is liable to come into contact with the metal that it is well insulated with paraffined paper or some other equally good material. Then wind the new wire on in smooth even layers and when it is completed fasten the bobbins back on the permanent magnets and connect them up. The current should flow through in opposite directions so that the north pole of one and the south pole of the other is on top. Do not trust splice connections but solder them using acid as a flux.

The Navy Department specifies that its wireless receivers shall be wound with copper wire of not less than 0.0015 inch in diameter and the diaphragm to have a diameter of 1 3/4 inches and a thickness of 0.004 inch. The resistance of the coils is specified at 1,000-1,100 ohms. There is not much advantage in greatly exceeding the number of turns possible with this winding, for to obtain them a much finer wire than No. 40 B. S. gauge is necessary and the ratio between resistance and turns becomes greater.

The second objection to the ordinary receiver is that the diaphragms are very often too thick. A receiver having a thin diaphragm is preferable because when a weak current is sent through the coils, the change in magnet strength is greater. But this may be carried to excess and the diaphragm made so thin that it cannot absorb sufficient lines of force to properly play its part. The best thickness then for a diaphragm can only be determined experimentally and depends much upon the diameter. The distance from the poles and the strength of the magnets will also have considerable bearing on the thickness. The ordinary phone will be very sensitive and give clear tones with diaphragms ranging from .01-.004 inch.