Crystal sets I have five home constructed crystal sets, two of which need crystal holders and cats whiskers and two others need coils. I have several spare coils so I am hoping to be able to get them all working in due course.
The picture above is one described in the BVWS bulletin of 1989 which closely resembles one of mine which I have been rebuilding. The article indicates that this was a designed in 1910 and further described in Popular Mechanics in 1918.The tuned circuit comprises two concentric coils the outer of which is 6 inches in diameter and 9.5 inches long. There are 348 turns and 29 tappings on the outer coil which are taken to the brass screws arranged in the upper semicircular switch arrangement. I do not know the diameter or details of the inner coil but the above mentioned article indicates that there are in total more than 1000 turns. I guess therefore that the inner one has about 650 turns with tappings every 50 turns.There are two variable capacitors, the one on the top made from zinc plates and pieces of glass is about 350pF and the other (bottom left) made from two pieces of brass tube is perhaps 20pF. The aforementioned article also indicates that the "long distance receiving apparatus" was designed to cover wavelengths from 7500metres down to 300 metres (40kHz-1MHz)
I have been experimenting trying to figure out how it should be connected up. You will see that I have connected a germanium diode across the crystal holder and as yet have not constructed a fixed capacitor which was missing when I acquired what was a rather dirty and corroded junk item. The connections between the coil tappings and the switch studs have all been replaced with tinned copper wire. It was also missing knobs on the switch sliders and the insulation on the sliding tube was a sticky length of impregnated fabric tape.
I was puzzled by the arrangement of the two wipers on the top switch, the shorter of which traverses about 12 of the tappings and the outer can traverse all of them if lifted over the shorter one. This is not possible with the design shown in the drawing above.
Have you any ideas or suggestions as to how it was intended to be connected up? From one of the numerous diagrams I found on this very informative site http://www.crystalradio.net/ I guess that it was intended to be wired up as indicated in the diagram below with the tubular variable capacitor in the aerial circuit. On this site there is a useful calculator for determining coil inductances by inputting the number of turn and the diameter of the coil former. From this and my own calculations it would seem that the primary is about 8500 microhenries and the secondary about 25000 microhenries. If my diagram is correct a degree of variable coupling is accomplished by altering the number of primary turns and their relative position with respect to the secondary using the top two switch wipers. For the higher frequencies the secondary can be switched completely out of circuit and the lower part of the primary is then the secondary tuned circuit.
Since these photos were taken I have modified the variable capacitor to be more like that shown in the drawing.
I have found it useful when experimenting with this and other crystal radios to connect a self powered loudspeaker in parallel with the SG Brown headphones. see here. A slight tap on one of the earpieces will switch the amplifier on.
The crystal set below is clearly designed to receive medium an short wave signals.
It appears to have been constructed using parts from some precision laboratory instrument and housed in the box that perhaps once held a meter. Note the home made capacitor marked .0003 connected across the phone terminals.
The pictures below are of two other home constructed sets which still need to be worked on.
