The letter radio can be mailed out of town for six cents. It is tuned by moving a spring clip from one coil tap to another.

HERE is a novelty radio receiver that you can slip into an ordinary envelope and mail to one of your radio-minded friends as an amusing birthday or holiday greeting. Mounted on a conventional scenic post card and covered with a second post card so that all wiring is concealed, the set consists of a simplified tuning coil and a crystal detector. With earphones clipped to two of its paper-fastener terminals and a ground and antenna attached to the two remaining terminals, the set is ready to bring in near-by broadcasting stations. Tuning is accomplished by fastening a small spring clip to the various taps on the homemade "spider-web" tuning coil, while a sensitive spot on the crystal can be found by shifting the cat whisker from one point to another until a station is heard in the earphones. The tuning coil is made by interlacing: 100 turns of No. 30 double-cotton-covered wire around seven "spokes" cut in a 33/4" diameter cardboard disk, as shown in the drawing below. The coil should be tapped every twenty turns by removing the insulation and applying a blob of solder. The two post cards, which form the "chassis" and "cabinet" of the tiny receiver are held together by the paper-fastener terminals. To save space, flat copper ribbon was used in making the various connections.

The tiny receiver with its postcard "cabinet" removed to show the wiring. Circuit and coil are shown at right.