Radio broadcast .. (1922-30)

In the R. B. Lab. 73 An ultra efficient homemade inductance. It is easily made and will improve the operation of many sets you will operate it. This gives you the required voltage drop. The current consumption of the tube in amperes, at the correct A-battery voltage, will also be found in the operating directions. Divide the required voltage drop by the current. The result is the minimum resistance that will permit the most efficient operation of your tube. For instance: Operating a Cunningham C-3OI-A from a six-volt storage battery. The correct operating potential for this tube is five volts. 6 — 5 = i — the required voltage drop is one. The 0-30 1 -A is a quarter-ampere tube, therefore, 1^-5=4 — i.e., at least four ohms should be used. Thus a six or ten-ohm rheostat will be sufficient. In cases where the adjustment of the filament temperature is at all critical (using the uv-20i-A as a detector in regenerative circuits, for instance) the lower resistances will permit a finer variation of current. The inter-relation of volts, amperes, and ohms, in regard to filament resistances and A batteries, will be found treated with especial regard to the principle of this very fundamental law in the October 1923 issue of RADIO BROADCAST. A NEW-TYPE HOME-MADE INDUCTANCE IN A recent issue of the Lab Department, we stated that the ideal inductance would be a self-supporting coil wound with uninsulated wire on air. Like many ideals, this arrangement is hardly practicable. Nevertheless, it can be approached, and in Fig. 8 we have what is probably the closest practical approach to this ideal condition, a coil wound by one of our readers, Mr. Horace A. Woodward, of New York City. The Sickles coil is a commercial form of this type of winding. It is essentially an exaggerated honeycomb. The winding form is a disk of wood about three inches in diameter and three quarters of an inch wide. Into the periphery of the disk, one eighth inch from each edge, two rows of twenty-five evenly spaced pins are driven. Two-inch, No. 14 finishing nails are convenient for this purpose. Notches, which facilitate the last part of the work, should be cut between the pegs (Fig. 9) with a three-cornered file. The coil is wound by passing the wire over two right-hand pins, diagonally across and over two left-hand pins as illustrated in Fig. 9. When the last turn is wound, the coil is sewn with a waxed thread and a flexible needle made of a short length of twisted wire. The needle is passed beneath the coil through the filed notches, taking the direction shown by the black thread in the photograph. If the NOTCHES •-'--'' FIG. 9 The winding form for the low-capacity coil experimenter prefers, collodion may be used as a binder and the sewing dispensed with, though this is theoretically inferior to the method employed by Mr. Woodward. The nails are finally removed and the coil slipped off. The inductance is self-supporting and will withstand an extraordinary amount of mechanical abuse. The ingenuity of the individual experimenter will suggest the most convenient manner of mounting. These coils may be substituted for singlelayer inductances in any circuit with probably an increase in efficiency. Mr. Woodward finds them decidedly superior to the spiderweb coils in the Roberts set. Assuming a threeinch diameter for the usual flat wound coils, the same number of turns on the improved inductance will give approximately the same wave range. BUILDING YOUR OWN LAB ONCE again we are rather prodigal, and for November we recommend two purchases to the owner of the growing lab — an automatic center punch and an adjustable square, shown in photographs Figs.