Page:Popular Science Monthly Volume 88.djvu/488

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FTER many years of scientific experimentation with various kinds of detector stands and contacts, the crystal detector stand described below was found satisfactory.

The detector is of the ferro type, mounted on a small box containing an arrangement for moving the crystal. The chief merit of the instrument lies in the use of a direct differential screw which insures perfect alinement.

This differential screw consists of a

combination of an 8-32 screw and a 2-56 screw. It is made by drilling in the end of the 8-32 screw a hole to be tapped out with a 2-56 thread. A little patience is required, for if the builder does not center the hole perfectly the first time, he must try again. This is where the success of the instrument lies. A 2-56 screw is to be screwed tightly into this hole.

A hole should then be drilled in the piece A, directly below the one in the piece B, and tapped out for a 2-56 thread. When the differential screw is screwed in place and turned one complete revolution, it will lower 1-32″, but at the same time the piece A will be raised 1-56″ by the other screw, or in other words, the piece A will be lowered 3/224″.

The rest of the standard is easily made, as may be seen from the diagram, the pieces A and B being of 1/16″ brass, ½″ wide and of a suitable length. The pieces C are of thin phosphor bronze or brass ½″ wide and of suitable length, and the pillars D of round or hexagonal brass of any size to please the maker. The parts are held together with 8-32 machine screws.

A box of ¼″ oak about 5 ½″ by 3″ by 1 ¼″ should then be made, the detector being mounted centrally on the cover. Two binding posts are put on the front as well as the knob for the adjusting arrangement. As will be observed the adjusting arrangement includes a screw with adjusting knob, the screw running through the box, and the far end having the thread filed off and resting in a small hole bored in the back of the box. Jam nuts are used in back of the front piece to prevent the screw from coming out.

A small guide rod guides the piece H, which is moved by turning the screw. The rest of the arrangement can be seen from the diagram. The disk cup should be mounted eccentrically on the shaft N so as to obtain a greater range and contact for the mineral. The hole in the top of the box should be small enough so that the mineral will be held firmly. The standard and mineral adjusting device are connected respectively to the two binding posts.

For a contact wire, a piece of very stiff German silver or other resistance wire (not iron) should be used, and heavy or light contact can be made with the point. No. 30 B. & S. is all right.



MATEURS often wonder how they can reduce the number of taps on their loose-coupler and still tune accurately. The following scheme has been used successfully and it is found that it not only saves money but the instrument works more easily. Instead of tapping every wire in the first ten and then every tenth wire, a tap is taken from every second wire in the first twenty and after that every twentieth wire. Thus half of the taps are done away with on the second switch. This, of course, only tunes in steps of two. To remedy this a separate single turn is added at one end of the primary and arranged with an extra two-point switch, so that this turn can be used whenever needed. With this arrangement, any possible number of turns may be used and the trouble and expense of making several extra taps are saved. Another advantage is that rough tuning can be accomplished much more quickly and, after a station is located, the more accurate tuning can be done.

