Radio Broadcast (May-Oct 1922)

50 RADIO BROADCAST After all the parts of this crystal-detector radio receiving set have been constructed and assembled the first essential operation is to adjust the little piece of wire, which rests lightly on the crystal, to a sensitive point. This may be accomplished in several different ways; the use of a miniature buzzer transmitter is very satisfactory. Assuming that the most sensitive point on the crystal has been found by method described in paragraph below, "The Test Buzzer," the rest of the operation is to get the radio receiving set in resonance or in tune with the station from which one wishes to hear messages. The tuning of the receiving set is attained by adjusting the inductance of the tuner. That is, one or both of the switch arms are rotated until the proper number of turns of wire of the tuner are made a part of the metallic circuit between the antenna and ground, so that together with the capacity of the antenna the receiving circuit is in resonance with the particular transmitting station. It will be remembered that there are lo turns of wire between each of the first 8 switch contacts and only i turn of wire between each 2 of the other contacts. The tuning of the receiving set is best accomplished by setting the righthand switch arm on contact (i) and rotating the left-hand switch arm over all its contacts. If the desired signals are not heard, move the right-hand switch arm to contact (2) and again rotate the left-hand switch arm throughout its range. Proceed in this manner until the desired signals are heard. It will be advantageous for the one using this radio receiving equipment to find out the wave frequencies (wave lengths) used by the several radio transmitting stations in his immediatevicinity. The Test Buzzer (Z, Fig. 3). — As mentioned previously, it is easy to find the more sensitive spots on the crystal by using a test buzzer. The test buzzer is used as a miniature local transmitting set. When connected to the receiving set as shown at Z, Fig. 3, the current produced by the buzzer will be converted into sound by the telephone receivers and the crystal, the loudness of the sound depending on what part of the crystal is in contact with the fine wire. To find the most sensitive spot connect the test buzzer to the receiving set as directed, close the switch (5, Fig. 3) (and if necessary adjust the buzzer armature so that a clear note is emitted by the buzzer), set the right-hand switch arm on contact point No. 8, fasten the telephone receivers to the binding posts marked "phones," loose the set screw of the binding post slightly and change the position of the fine wire (6, Fig. 3) to several positions of contact with the crystal until the loudest sound is heard in the phones, then tighten the binding post set screw (4) slightly. APPROXIMATE COST OF PARTS THE following list shows the approximate cost of the parts used in the construction of this radio receiving station. The total cost will depend largely on the kind of apparatus purchased and on the number of parts constructed at home. Antenna Wire — Copper, bare or insulated. No. 14, TOO to 150 ft., about 0.75 Rope — I or J inch, 2 cents per foot 2 Insulators, porcelain . . . 0.20 I Pulley 0.15 Lightning switch — 30 ampere battery switch . . . . 0.30 I Porcelain tube o. 10 Ground connections. Wire (same kind as antenna wire) I Clamp 0.15 I Iron pipe or rod . 0.15 ^ pound No. 24 copper wire double cotton covered . . . . 0.75 1 Cardboard box .... 2 Switch knobs and blades com plete 1 . 00 18 Switch contacts and nuts . . 0.75 3 Binding posts — set -screw type 0.45 2 Binding posts — any type . . 0.30 1 Crystal — tested 0.25 3 Wood screws, brass, f inch long 0 .03 Wood for panels (from packing box) 2 Pounds paraffin 0.30 Lamp cord, 2 to 3 cents per foot Test buzzer o. 50 Dry battery 0.30 Telephone receivers . . . . 4.00 to 8.00 * Total .11.00 15.00 If nothing but the antenna wire, lightning switch, porcelain tube, crystal, telephone receiver, bolts and buzzer are purchased this total can be reduced to about ^6.00. *Still more efficient and expensive telephone receivers are available at prices ranging to about $20.00.