Page:Popular Science Monthly Volume 91.djvu/173

 The Construction of a Magnetic Break Key

��With it the lightest kind of a Morse key may be used for sending

By T. Lambert

��THE average experimenter's "break key" consists of a number of springs, contacts, etc., which are attached to his regular transmitting key in a clumsy manner. To send clearly a code with key contacts 34 in- apart is next to impossible. With the relay-key described herein it is possible to use the lightest kind of a Morse key for sending, since all the clumsy con- tacts of the breaks are on the heavier magnetic key. Fur- ther, no heavy cur- rent is handled by the small key and thereby arcing and sticking are eliminated.

A key as described has been in use in my amateur station for some time and has never given any trou- ble in holding the detector -adjustment, even with the most sensitive minerals, such as galena and cerusite.

The completed in- strument is shown in the illustration. Four spring-contacts are employed, besides the alternating current break. The pair at the right disconnect the detector from the receiver when depressed. One of the contacts at the left shorts the telephones while the other one grounds the entire receiver while sending. It will be seen from diagram B that the receiving- tuner's primary is placed in series in the ground-lead of the oscillation-trans- former.

When sending, the high voltage currents from the transmitter pass directly across

���Four contact springs besides the A. C. break are employed in this magnetic break key

��to the ground through the last mentioned contacts on the key. Some stray current may find its way into the receiver but will not damage either telephones or detector because they are amply protected by the other contacts. There is absolutely no danger of shock as all receiving apparatus is on the grounded side of the oscillation-trans- former. The magnet cores are shown at C. A good grade of soft iron should be used, as it is less liable to hold residual magnet- ism. The cores are threaded to fit the end pieces snugly. The windings can be made directly on the cores after a layer of tape is placed, but it is preferable for ease in winding to turn out two bobbins on a lathe, as shown at D, and wind on them. Use number 20 D. C. C. wire and wind on eight or ten layers, placing a heavy sheet of paper between the layers. On the final layer glue a thin piece of ebonite to give it a finished appearance. The brass crosspiece which holds the screw for adjustment of the tension of the spring E (page 159) is shown at F. The piece G is of hard rubber and sup- ports the stationary primary contact as well as the yoke H for regulating the play of the key. It is attached to the top of the magnets with two machine-screws. The armature support is cut from J^-in. brass. The contact and crosspiece holes are best located after it has been set up on its pivot J. This is also of fcrass, cut to the

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