Variometer and Crystal Broadcast Receiver
Fig 1 shows a simple variometer crystal set which gives good results up to 20 miles from a broadcasting station with the ordinary single or twin wire outdoor aerial. The set has the advantage of being very compact, the overall dimensions being 6 1/2 in. by 3 1/2 in., and being cheap to make.
The materials required are:
Baseboard, 6 1/2 in by 3 1/2 in., by 1/2 in; large cardboard tube, 2 3/4 in. wide, 3 1/2 in. diameter, small cardboard tube 2 1/2 in wide, 1 3/4 in. diameter; one glass encased detector; one ebonite knob; 3 in. of 2 B.A. screwed rod; 45 ft 28-gauge S.C.C. wire; four terminals, nuts and screws and washers.
The baseboard should be made first, as when it is varnished it can be put aside to dry while the remainder of the set is being constructed. The board is made from any suitable wood, oak, mahogany, or beech being specially recommended. When cut to size and edges squared, a bevel is made on the top edge, or if a moulding plane is available, a better finish can be given by its use. Four strips of wood 1/2 in. square, are glued flush with the sides on the underneath of the base. This brings the base to 1 in. thick, giving a very substantial appearance and obviating the necessity of recessing the nuts and channelling grooves for connexions under the base. When these strips are quite firm the whole base is planed up, sandpapered, and varnished.
A variometer consists essentially of two parts, an outer inductance and an inner one. In the large majority of cases, and owing to natural ease of construction, the inner inductance is capable of rotation inside the larger one, which is stationary.
For this reason they are respectively called rotor and stator. The stator is cut from cardboard tube of 3 1/2 in. diameter and a width of 2 3/4 in. The only process demanding exact workmanship is in drilling the two holes in both rotor and stator so that the former will readily rotate inside the latter without touching it. The process adopted for both is exactly the same, so that only one need be described in detail. Drill a 3/16 in. hole centrally in the stator. This will be 1 3/8 in from the center to either edge.
A strip of paper is cut about 1 in. wide and a little over half the length of the circumference of the stator. One end is to the outside of the hole just drilled and the paper then wrapped round. At a point judged as half-way round, a pencil mark is made from the paper to the stator. The same process is carried out on opposite side of the stator.
It will be seen that the two pencil marks made on the stator represent an. equal distance from the centre of the, hole, and the midway point between the two marks gives the centre of the hole to be drilled. Proceed in this way with the rotor, both holes again being 3/16 in. diameter. In order to give a more pleasing appearance and to make the cardboard look like ebonite, the rotor and stator may be painted with a black enamel inside and out before winding.
The stator is wound with 26 turns of No 26 S.C.C. wire. Commence winding 5/8 in. from the edge. When thirteen turns have been put on, the wire is taken straight across, not fouling the holes drilled, and rewinding commenced 1 in. from this edge.
The layers occupy about 3/8 in., so that this measurement will make the two half coils balance. When all the wire is on, two terminal holes near each edge are drilled in line with the centre hole.
The rotor has 26 turns of the same wire on each side of the central holes. Winding, is commenced 3/16 in. from one edge and finished off the same distance on the other side. The outsides of both rotor and stator are given two coats of white hard varnish to fix the windings permanently into position. Two more holes are required in the stator to hold the complete variometer in position, and are drilled in line from each edge at a distance of 3 3/4 in. along the circumference from the terminal holes. All the holes in rotor and stator are bushed with brass eyelets. They should have a central hole of 3/16 in.
These are shown on the rotor and stator in Fig. 2. The 3/16 in. holes already drilled will need to be slightly enlarged for the eyelets to fit. The tang of a file will be sufficient to open the holes out. The eyelets are then pushed through the holes with the rounded side on the inside. Both ends of the rotor connexions are fastened under their two eyelets before bending over the tag ends. In the stator one wire is fastened to the central eyelet opposite to the three holes in line.
The remaining stator wire is held under the right eyelet of the three inline. This terminal is seen on the stator in fig 2 of the complete instrument.
The longer piece is slipped through the hole between the terminal holes and a 2 B.A. spring washer pushed on from the inside. This washer is followed by a 2 B.A. lock nut, after which the rotor is slipped on. Another 2 B.A. lock nut screwed on from inside the rotor will bind up the spindle tight with the rotor.
Do not allow any spindle to project inside the rotor or it may be too short to take a nut and knob which will be fitted later. Exactly the same procedure is carried out on us opposite side with the shorter length of screwed rod. A spring washer and nuts are fitted in the same order. Each spindle now receives a nut on the outside. They must be tightened up to bring the rotor in the middle of the stator, so that the distance separating both. is the same all the way round. When this is so another lock nut is screwed on each spindle.
The base is drilled and is shown completed in Fig. 3. A glass covered detector is fitted as in Fig. 4. The holes for mounting the detector are not shown in Fig. A, as they must vary according to the type of detector purchased., Mount the variometer in position with two 2 B.A. screws, with nuts 1 in. long. An ebonite knob is screwed on to the longer stemming and tightened up by a lock nut screwed on first. Two terminals are fitted to the adjoining holes, wires clamped under the bases going through two small holes immediately underneath them.
The wire on the left goes directly to a terminal on the left of the baseboard, E in Fig A. The wire on the right goes to the terminal F on the right of the baseboard. (Fig A) after passing through the crystal detector. The wiring under the base is shown. Before the set is finally finished the nut under the rotor, where the longer spindle is held, should be given another tightening with a small spanner, Fig 5. This nut bears all the strain of the rotors movement and it is important it should be quite tight. The terminals on the variometer from left to right are, respectively, aerial and earth. The terminals on the base are telephone terminals.
The central eyelet is connected to that on the left by a short length of wire. For assembling the rotor to the stator two lengths of 2 BA rod will be required , 1 3/4 in and 1 1/4 in respectively.
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