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 



ANTENNA 



DETECTOR 



VAR. / 



y COHO. J •• PHONES 



V COND. Q^ 



FIG. 2 



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 



/^ 



I ._<>. 



fl6 5 fl6.4 



The form of the plates are arcs 

 of circles having a 2-in. radius 



fl6.5 



(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 



