ELECTROLYTIC DETECTORS.
"Bare Point" Type.—Although the electrolytic is the oldest of a long line of very sensitive detectors,[3] it still holds first rank when in the hands of an experienced and skillful operator. It exists in two different forms, but the more favored is that known as the Fessenden "bare point" type, which consists of a very fine Woolaston platinum wire (.001-.00002 of an inch in diameter) dipping in a small cup of dilute acid. The acid is either 20 per cent chemically pure nitric or sulphuric.
Fig. 91. Bare Point Electrolytic Detector.
A large electrode of platinum wire or foil dips into or is sealed in the bottom of the cup so as to make an electrical connection with the liquid. The fine Woolaston wire is clamped over the cup in a holder which permits of vertical adjustment, by means of a thumbscrew, so that the depth of immersion in the acid may be regulated.
Fig. 92. Effect of Exposing too much Wire.
Woolaston wire is covered with a comparatively thick coating of silver, which before using must be removed from the end for about 1/32 inch by dipping it in strong nitric acid, which will dissolve the silver and expose the almost invisible platinum core. Too much of the fine platinum core must not be exposed or else the surface tension of the acid will cause the wire to curl over and present a large flat surface instead of a fine point. This is a very necessary and important precaution, for the detector is more sensitive as the area of contact between the fine wire and the liquid is smaller.
Whenever this condition is reached the end of the wire should be cut off with a pair of sharp scissors and a new point exposed.
Fig. 93. Electrolytic Detector Circuits.
The detector circuit is shown in Fig. 93. The fine "bare point" is always made the positive or anode of the battery circuit. Otherwise the detector will not operate. A potentiometer must be shunted across the terminals of the battery to reduce the voltage to a value just below that which is required to break down the thin film of oxygen gas which collects on the "bare point" and polarizes it or insulates it from the liquid so that little or no battery current can flow. This film of gas is caused by the electrolysis of the acid solution and the decomposition of the water into hydrogen and oxygen gas.
When oscillations are set up in the receiving aerial and they surge through the detector, a sufficient e.m.f. is generated to break down the film of gas and permit the battery current to flow again. The passage of current causes the signals in the telephone receivers.
The electrolytic cup for the universal detector is illustrated in Fig. 90. It is made of a piece of hard rubber rod 3/4 inch in diameter and 3/4 inch high. A recess 1/2 inch in diameter and 3/8 inch deep is cut in the top to contain the acid. A small hole 1/4 inch deep is bored in the under side and threaded with an 8-32 tap. A brass pin 1/2 inch long, having a corresponding thread, is fitted in the hole. The pin may then be screwed into the small hole in the bedplate. A piece of No. 30 B. S. gauge platinum wire or a strip of platinum foil is clamped between the bottom of the cup and the bedplate and then bent over the top of the cup into the liquid.
Fig. 94. Electrolytic Detector.
A 1/16-inch hole 1/4 inch deep is bored in the lower end of a thumbscrew having an 8-32 thread. A piece of Woolaston wire 1/2 inch long is placed in the center of the hole and tinfoil packed into the surrounding space with the head of a sewing needle until the wire is held firmly in position. The free end of the wire must then be dipped in some strong nitric acid to remove the silver. The thumbscrew is placed in the collar on the end of the spring of the universal detector and lowered until the "bare point" almost touches the surface of the electrolyte in the cup beneath. Pressure must then be applied to the spring by turning the large adjusting screw until the "bare point" touches the liquid and a click is heard in the telephone receivers and a faint bubbling sound is also audible. The adjusting screw must then be slowly and carefully turned in the opposite direction so as to raise the point until the bubbling changes to a hissing sound. The point is then above the level of the electrolyte in the cup but is still in contact with it because of the capillary action of the fine wire and the liquid.
By using the large adjusting screw as much as possible, instead of the small thumbscrew, the point is raised or lowered without giving it a circular motion and much finer adjustment is made possible. The potentiometer is adjusted until the hissing noise caused by excessive battery voltage just disappears. The detector is then in its most sensitive condition for receiving signals.
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.
Fig. 97. Shoemaker Detector Circuits.
The Shoemaker system makes use of a loop aerial, and the circuits with a single and double coil tuner are illustrated in Fig. 97. It is not necessary to use these, and the detector will operate just exactly as well on a "straightaway" aerial.
Lamp Detector.—All electrolytic detectors, more especially those of the "glass point" type, are subject to the annoyance of "burn-outs." That is, the fine platinum wire melts when receiving strong signals from a near-by station.
In such case, the "bare point" must be lowered until it again makes contact with the liquid, and the "glass point" reground until the wire is again exposed.
When this trouble comes often it is very convenient to have at hand a simple detector which will not burn out and which may be substituted for the usual one when great sensitiveness is not required.
Fig. 98. Lamp Detector.
Such an instrument is made by snipping off the tip of a small incandescent electric lamp and removing the filament with a wire. One of the leading-in wires is broken off as close as possible to the glass stub and the globe half filled with a 20 per cent acid solution. The broken wire must be made the negative or cathode and connected like a Fessenden or Stone detector. This lamp detector though crude will give good service without burning out when used to receive from near-by stations.
In place of a lamp detector, a glass point having a larger wire than that of the cigar lighter may be used instead of the usual point, but it will not be so sensitive.
Fig. 99 illustrates a simple form of electrolytic detector which is not so sensitive as that shown in Fig. 100 but is still very serviceable.
The cup is made from the carbon of an old dry cell, the brass connecting cap serving very well to make the connections to. It has a recess about 1/2 inch in diameter and 1/4 inch deep cut in the top to contain the electrolyte. The cup should be about one inch high. A file will smooth up any rough edges and give it a good appearance.
Fig. 99. Simple Electrolytic Detector.
The yoke is made of a piece of 1/8-inch sheet brass about 3/4 inch wide, bent in the shape shown in the illustration. Two small holes are drilled in the feet, to serve to fasten the yoke firmly to the base and also to make connection to.
The adjusting screw may be made from the screw taken from the carbon of an old dry cell. To permit of accurate adjustment, it should be fitted with a large head made from a piece of 1/4-inch hard rubber or fiber cut in a circle about 1 1/4 inches in diameter. Bore a small hole about 1/8 inch in diameter through the center of the head and force it on the screw. A nut screwed on the under side will then clamp it tightly against the brass head. A hole is bored in the center of the yoke and a battery nut which will fit the adjusting screw soldered directly under it.
The platinum wire may be either soldered to the adjusting screw or fastened with tinfoil in the method which has been described.
The cup and yoke are best mounted on a piece of hard rubber 1/2 inch thick, 3 inches wide and 4 inches long. A binding post is placed near each of the four corners.
It is possible to do extremely fine and long distance work with the detector illustrated in Fig. 100. It is so arranged that the "bare point" need not necessarily be revolved when making an adjustment, and so it is possible to place it in a very sensitive condition.
A brass standard, U, 1 1/4 inches long is cut from a piece of 1/2-inch rod. A hole bored in the top and bottom of the standard is threaded with an 8-32 tap. A brass rod, R, 2 inches long is threaded with an 8-32 die throughout its entire length. One end is screwed in the top of U.
A piece of brass tubing, P, 1 1/4 inches long and having an internal bore of 1/2 inch is slipped over U. A slot cut in P fits over a small pin set in U and permits P to be slid up and down but not to turn around.
Fig. 100. Electrolytic Detector.
A head in the form of a circular brass washer, E, 1/8 inch thick, 1/2 inch in diameter and having a 5/32-inch hole bored in the center is soldered in the top of the tube, P.
A circular piece of hard rubber, H, 2 inches in diameter and 1/2 inch thick is fitted with a brass bushing having a hole in the center with an 8-32 thread to screw on the rod, R.
A spiral spring is placed around R between U and the head E. A small brass washer should be placed between H and E in order to eliminate friction. When H is turned in one direction, the spring will cause P to rise, and when turned in the other direction it will be lowered.
A brass arm, A, 1/4 x 1/4 x 1 1/2 inches carries a small thumbscrew, r, at one end, while the other end is soldered to P as shown in Fig. 96. The Woolaston wire is soldered to T.
Fig. 101. Details of Electrolytic Detector.
A small carbon cup 3/4 x 3/4 inch serves to hold the electrolyte. A 3/8-inch hole is bored 1/4 inch deep in the bottom of the cup and poured full of melted lead. The lead is then bored and tapped to fit a machine screw which fastens the cup to the base. Connection is made from a binding post to the machine screw. A second binding post is connected to the screw which fits into the bottom of U and holds it to the base.
If desirable a circular piece of hard wood, F, may be turned out and glued to the base around the cup in order to give it a more finished appearance.
The thumbscrew, T, is used to lower the "bare point" until it almost touches the liquid, and then the large head, H, is brought into play to make the finer adjustment.
Increasing the Sensitiveness of an Electrolytic Detector. The sensitiveness of an electrolytic detector may be increased in three ways, viz., by connecting two detectors in series, by warming the electrolyte and by agitating it.
The first method is clearly apparent.
The second is accomplished by placing the detector over a sand bath and gently warming it. It will then show a marked increase in the strength of the signals at a temperature of about 30 C. This increase will continue to rise with the temperature until it reaches a maximum at about 60 C.
Fig. 102. Increasing the Sensitiveness of an Electrolytic Detector.
Branly discovered that a fine stream of gas passed through the electrolyte in order to agitate it increases the strength of the signals in the phones. He devised a detector provided with two extra platinum terminals sealed in the cup. When connected in series with a battery and an adjustable resistance, these terminals cause electrolysis of the water, and a fine stream of oxygen and hydrogen gas flows through the acid electrolyte. The stream of gas agitates the liquid just sufficiently so that when oscillations strike the detector they augment the breaking down of the film of gas which collects on the fine platinum point. This results in an increase in the battery current flowing through the telephone receivers of from two to four times and a corresponding increase in the volume of sound. The adjustable resistance is used to regulate the decomposition of the electrolyte and formation of gas, for if this proceeds too rapidly an undesirable rumbling noise will be produced in the telephone receivers.