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Page:Popular Science Monthly Volume 90.djvu/492

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476

��Popular Science Monthly

��which appear from time to time, are less likely to be satisfactory.

Since good variable condensers are ex- pensive, as compared with the other parts of short-wave receiving apparatus, a simple and yet good design for making them at

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��Diagram for inductively coupled receiver with variable condenser and single pole switch

home will be described. The condenser made in this manner will prove rugged and suitable for continuous operation, will have good insulation between its terminals, and yet will be found easy and comparatively quick to build.

The Fixed and Moving Plates

The plates of the condenser may be made of almost any conducting material. Alum- inum about 1/32 in. in thickness is by far the best, since it is light, soft and easily kept flat. Soft brass or copper will prove suitable though heavy, and even sheet tin can be used if reducing the expense is of the greatest importance. The fixed plates are laid out as shown in Fig. 3. They con- sist merely of rectangles 2 by 5 in. in size. About ^ in. from each corner, on a 45 deg. line a hole is drilled to take the support- ing uprights. The relation of the plates to a semi-circle of 2 in. radius, which corre- sponds to the active surface of the moving plates, is also shown in Fig. 3. The rectan- gular form of plate is shown for the reason that it is the quickest and easiest to cut out. Obviously, if material is of more importance than time, the corners may be cut off and the outer side of the plate held by a single vertical bolt passing through suitably placed holes at or near the center of the upper edge.

The form of the moving plates is shown in Fig. 4. Essentially, these are portions of circles having a 2-in. radius; they are roughly semi-circular in form, and the exact relation to a half-circle is shown by the dotted lines. A hole to take a 5/32-in.

��bolt is drilled at the central point where the radii meet, as shown. This design of plate is about the easiest to make of all that have been suggested, and yet is not particularly wasteful of material.

For the tuning condenser, 13 fixed and 12 movable plates may be cut out. The sim- plest way is to make a full size templet or pattern out of pressboard, and to scratch the outline of each plate on the material by running a needle around the edge of the pattern when held tightly on the surface of the metal sheet. The plates are then trimmed out with sharp shears, clamped together in a vise and filed to exactly the same size and shape. They must be flattened by hammering, preferably be- tween perfectly flat metal surfaces, until no dents or warping can be seen when the plate is held edgewise to the eye.

The Top and Base

Hard rubber or horn fiber, from 3^ to^ in. in thickness is the best material for the top and base. If these cannot be used, hard- wood about 3^ in. thick will do. In the illustration. Fig. 5, are shown the tv/o pieces that form a 5-in. square, and how they are drilled at each corner, in the center, and at the inner corners of the fixed plates, to take the several bolts. The base must also be drilled and tapped near the center, to take the foot or base bearing of the vertical shaft, to be described later. In Fig. 5 there are shown the outlines of both the fixed and moving plates in their proper relative posi- tions, so that no difficulty should be ex- perienced in laying out the holes once the plates have been finished.

Other Materials

Practically nothing else is needed except - a number of 8-32-hexagon brass nuts -M

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The form of the plates are arcs of circles having a 2-in. radius

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��(machine screw size No. 8 with 32 threads per inch), a considerable quantity of copper washers or burrs which will slip freely (but not too loosely) over an 8-32-machine screw, six threaded brass rods of 8-32 size and about 5 in. long (the excess is cut off^ after assembly), a 5 in. by 5/32 in. threaded

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