Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

When the detector is in use for long periods, the potentiometer must be frequently readjusted to compensate the gradual loss in voltage of the battery. It is well to provide a small switch which will disconnect the battery from the potentiometer when the detector is not in use. In the same case the acid should be removed and placed in a tightly stoppered bottle. A pipette or fountain pen filler furnishes the most convenient means for filling or emptying the cup. The acid must be kept perfectly pure and out of contact with all metals other than platinum. Great care should be exercised in filling the cup, for the acid, if spilled, will not only badly corrode the metal fittings, but will also provide a current leak and seriously weaken the signals.

Shoemaker and Stone Detectors.—These two types of detectors make use of "glass points," so called because the fine platinum wire is sealed in a glass tube and the end of the tube is then ground down on a fine oilstone until the platinum wire is exposed. This results in a very fine contact area and insures constant immersion of the point without readjustment.

Fig. 95. Forming "Glass" Point.

The fine platinum wire for a glass point may be secured from one of the flaming pocket cigar lighters making use of spongy platinum. The center of a thick walled glass tube is softened by heat and contracted as shown at A-B in Fig. 95. After cooling, the tube is cut in half at the point indicated by the dotted line. The platinum wire is placed in the contracted end of the tube and carefully fused in so that about one-half of the wire, which is about 1/2 inch long, is embedded in the glass. The contracted end of the tube containing the wire should be closed. Connection is established to the upper end of the fine platinum wire by filling the tube with mercury and dipping a piece of flexible conductor in the mercury. The upper end of the tube is closed and the mercury prevented from escaping by a small dab of sealing wax.

The "point" is slowly and carefully rubbed on a fine oilstone kept well wet with water. The tube must be held in a vertical position so that the glass will be ground away at right angles. When it is thought that the platinum wire has been exposed by the grinding, connect the flexible conductor to one pole of a battery. The other pole of the battery is connected to a pair of sensitive telephone receivers and the telephone receivers to a vessel containing dilute acid. If the platinum wire is exposed, a sharp click will be heard in the telephone receivers when the "point" is dipped in the acid. Do not confuse the sharp click with the sound which may be occasioned because the outside of the glass tube is damp or wet.

After the point has been sufficiently ground, disconnect the testing apparatus and connect the free end of the flexible conductor to a binding post placed on the end of the detector spring, S. The detector circuit is similar to that of the "bare point" type.

Fig. 96. Shoemaker Detector.

The illustration shows what is sometimes called a "primary cell" detector because it furnishes its own current and does not require a battery. A Stone detector may be very easily changed to one of the Shoemaker type by substituting an amalgamated zinc rod for the platinum wire anode which makes connection with the liquid in the cup. This combination of platinum and zinc results in an electromotive force of about 0.7 volt, and the telephone receivers are connected directly to the terminals of the detector without any local battery or potentiometer. The electrolyte in the cup must be a 20 per cent solution of pure sulphuric acid, as nitric acid would dissolve the zinc in a very few minutes. The zinc must be kept well amalgamated with mercury.