Page:Popular Science Monthly Volume 90.djvu/163

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�Q Amateur - Electrician

��Tf\nd WIi'gIgss Operator

��A Perfected Slider for Tuning Coils in Aerial Apparatus

A CONTACT for tuning coils that actual- ly slides and does not scrape is illus- trated. Ordinary sliders make such un- reliable contact and wear out the wire so quickly that they have generally been discarded for the more cumbersome switches. This slider is useful for both

��Rubber suoeRn mtol roo



The metal gear-teeth mesh in the wire coils in contact like a rack and pinion

sending and receiving coils, though, of course, the dimensions will be different for each case.

The slider is formed like a gear wheel. The coil-turns serve as a rack, so that the net effect is an almost frictionless direct contact, turn by turn with the coil. Where an ordinary slider makes only line contact by friction with the wire, this slider contacts a larger portion of the conductor. The wire is wound in screw-threads cut in a lathe into a composition non-warping tube, so that the turns can be accurately spaced. The wheel is cut from copper or brass to match the coil pitch, though the less exacting builder may use an old alarm- clock wheel or even a knurled binding- post. A pitch of 20 with corresponding wheel about y% in. in diameter has been found best for receiving coils. For the inner coil of a loose coupler the wheel is arranged to contact through a slit in the tube from the inside, so that the secondary may be moved in or out of the primary'.

Of course a plain wheel may be sub- stituted, but all of the good qualities out-

��lined will not be retained. This simple improvement constitutes an important betterment of this class of variable coils. This, of course, is equally suited to rheo- stats and other helical coils. There is substantially no wear on the wire and consequently no short-circuiting of turns by burrs carried over by the erosion of a friction slider. — P. E. Edelman.

��Fastening Binding-Posts on the

  • Ends of Heavy Wire

TAKE an empty rifle or revolver shell, about a 32-caliber, and through the closed end punch a hole large enough to receive the screw of a binding-post. File the screw off short enough and then push it through the open end of the shell, through the hole in the closed end, and screw the parts of the binding-post in place. Clean the inside of the shell and fill with solder. Insert the end of the wire and heat well over a blow-torch flame. Use soldering flux.


��How to Make a Ten-Ampere Key for Wireless

THE contacts of the ordinary telegraph- key are not large enough to stand the current of a fair-sized transformer. But by adding the contacts as shown in the illustration any telegraph-key can be made of J^in. sheet brass bent as shown. The end of the stand A is drilled and tapped for a 5/32-in. brass ad- justing rod. 5, which is provided with a lock- nut and adjusting handle. At the end of the screw a small piece of silver — a dime will do — is soldered and another is soldered to the rear end of the key lever, as shown, with a hard solder. The upright A should l>e pro\nded with a binding-post, as the old contacts of the key are not used. — Edward Jasper,


��Ordinary telegraph-key with silver contacts


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