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Page:Popular Science Monthly Volume 90.djvu/332

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316

��Popular Science Monthly

��factories, when the starting rheostat gets out of order. For a lOO h. p. motor, a water rheostat of the dimensions given below will be quite satisfactory. Assuming that the voltage is no, so that the amperes on full load amount to 680, a fairly large-sized wooden cask is necessary, to allow for a proper dissemination of the heat which is produced by the heavy current. Ac- cordingly, a large wooden cask is filled with slightly acidulated water — sulphur- ic will do — and an iron plate having four square feet of exposed surface bolted to one side of the cask in the solution. Another plate of the same dimensions is lowered into the water with a heavy, flexible, insulated cable. When it is completely submerged, a short circuiting switch is thrown in.

��Hairspring of a Watch as a Detector-Point Support

THE illustration shows a home- made detector which may be help- ful to those who do not need to construct a more elaborate instrument.

The hairspring of an old watch pro- vides support for the point, and is an effective shock absorber. It is held by a drop of solder to the adjustment arm, which is bent at right angles after insert- ing it in its binding-post support.

A one-inch length of annunciator wire is sharpened to a point for the crystal contact. Wind a few of the innermost turns of the spring around it, as shown in the drawing, and fasten with a well-

���A length of wire pointed and supported by the hairspring for a crystal contact

placed drop of solder to avoid loose contact.

If the conventional cup-type mineral holder is not to be had, a good clamp can be made very easily by bending up one side of a small piece of sheet brass as shown. Solder a nut taking an 8/32 in. screw directly opposite, and grip the crystal by tightening the screw on it.

��Making a Miniature Light to Attach to a Book

��/y/re inju/ated 'Jww /amp jcreiTi

���M

��■Boo A coyer

A tiny electric light for attaching to book covers

��ANY a sum- mer evening would be more enjoy- able if one could sit on the veranda and read with only light enough to illuminate the book. The same is true in win- ter, when there is an open fireplace.

A simple book-light can be made by bending a strip of brass, y% in. thick and I in. wide, as shown in the diagram. Bore a hole large enough to receive a miniature lamp, which should fit snugly. Attach two thin strips of brass (3^ in. wide), to the other end of the first piece of brass to act as a spring, which can be slipped over the cover of the book. The wire connections are shown in the illustration. — Wm. E. Finkernagel.

An Airtight Quenched Spark-Gap

WHETHER or not quenched spark- gaps should be made completely airtight has often been a subject of dispute. The Telefunken gaps are usually fitted with plain mica separating- rings, and no great pains are taken to exclude air from the space between the sparking surfaces. In spite of this, the Telefunken trans- mitters as a rule op- erate efficiently and with uniform, clear spark-tones. The effectiveness with which a quenched- gap prevents "back- firing" depends to a [^ large extent upon the condition of the parallel sparking- surfaces. If these are perfectly smooth and very slightly oxidized, the operation of the apparatus is usually most satisfactory. How long the plates will remain in good condition depends upon the material from which

���Construction of De Forest's air-tight gap

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