The Variometer Crystal Set

    This simple crystal set is described in ‘The Boy’s Book of Wireless’ by Ernest H. Robinson, published by Cassell and Co. of London in 1923.

    I have built this set following closely the instructions and diagrams as set out by the author and have tried to make it look authentic by using materials similar to those available to a constructor in 1923. It would have been nice to wind the coils with cotton covered wire but it is difficult to find these days. Brass and nickel plate parts and an ‘Ebonite’ and wooden base board give that antique look.
 
    In the circuit diagram a 0.002 capacitor is shown across the headphones. The author suggests that this is optional and he leaves it out on the construction diagrams. I have found its inclusion makes no difference to the performance of the set as most magnetic headphones have ample self capacity.
 
    Using a variable inductor as the tuning element was common in early crystal set designs as it avoided the purchase of an expensive variable capacitor but it did have some drawbacks. Selectivity is broad and the tuning range is restricted, not covering the entire broadcast band. Remember though that in 1923 even a large city such as London had only one broadcast station so adjacent channel interference was not a problem! The theory of operation is that the inductor acts as a loading coil to bring the antenna to resonance, so the ‘tuned circuit’ is the inductor plus the capacitance of a short (less than a quarter wavelength) antenna. Things would probably not work too well if the antenna was longer than a quarter wavelength and inductive, but that would mean an antenna length greater than 75 meters or 250 feet. 

 
Construction Hints
    I buy most of my hardware and electronic bits and pieces from an Australian outlet called Jaycar Electronics. They do a very good mail order service and stock nearly everything you could ever need. They have a website and catalog in US dollars. Check out http://www.jaycarelectronics.com for the online catalog and to look at the parts I mention and http://www.jaycarelectronics.com/catalog to order a paper catalog .

 
Base Board
    The base board is made from 3mm thick black acrylic sheet and measures about 125mm x 175mm. I found some acrylic which was glossy on one side but matte on the other. The matte side looks like the traditional Ebonite board used in early radio equipment. I get my acrylic from a factory which makes plastic advertising signs. They always have cheap off-cuts. A 12mm x 12mm timber edging gives room under the base board for the wiring. A small wooden picture frame makes a ready made edging. Check out K-Mart or similar. NOTE – NEVER solder any connections to components while mounted on the acrylic base board – ACRYLIC WILL MELT! Use solder tags then bolt it all together.(Jaycar catalog number HP-1350)

 
Terminal Posts
    I make these from nickel plated brass 10mm long tapped spacers (Jaycar catalog number HP-0900). These spacers are tapped through with a standard 3mm metric thread. Drill a 2mm hole through the side of the spacer about 5mm from one end such that a wire placed through the hole will be held firmly by a screw screwed in from the end of the spacer. Get some 3mm screws to fit. (Jaycar catalog number HP-0400)

For an antique look, remove the nickel plating with fine sandpaper and polish up the underlying brass.

 
Cats Whisker Holder
    The diagram explains the construction pretty well. The brass strip and threaded rod came from a hobby shop. The brass strip measures 6mm wide by 0.6mm thick by 105mm long and was bent around a 25mm diameter piece of PVC pipe. The threaded rod is about 40mm long. The brass adjusting nuts/knobs came from the tops of old dry cell telephone batteries. You will have to hunt through your junk box for something similar. The catswhisker is fine bare copper wire soldered in place.

Crystal Holder
    The crystal holder is made from a cut down male and female ‘F’ connector. This type of connector is used on RG-6 and RG-59 TV coaxial cable. What you need here is an F59 screw on plug (male) and a F61 chassis or wall plate socket (female).Have a look at the pictures in the Jaycar catalog. (Jaycar catalog numbers PP-0637 and PS-0645).  Start with the male plug. What we want from here is the threaded coupling ring. Slip a small hacksaw blade behind the coupling ring and carefully cut the body of the plug in half. This will allow the ring to fall free. Now take the female socket and using a pair of pliers, pull out the connector pin from the back of the nylon center of the socket. Take twist drill that will fit into the front of the socket and drill a shallow hollow in the nylon center of the socket. Do not drill away any metal. Fit a suitably sized piece of crystal (Galena) into the front of the socket that you have just drilled out and secure it by screwing on the coupling ring salvaged from the male F connector plug. Mount the crystal holder through the base board and secure with the nut and washer supplied. A solder connection is made to the side of the washer before mounting. (otherwise you will melt the acrylic base board). By the way, there is a good description on this website about how to make Galena crystals using lead, sulphur and your mother’s best tea spoons – but don’t tell her. (See Fun Tips and Projects – page 2)

 
Spiderweb Coils
    How to wind spiderweb coils is described elsewhere on this website. The diagram gives the dimensions of the cardboard formers which in this case are not removed when the coils are finished. Be careful that both coils are wound and mounted so that the wire turns go in the same direction. I wound 32 turns of plastic insulated wire on each of my coils, as much as I could get on the formers. Varnished winding wire will allow more turns.  The original notes suggested 40 turns but the exact number to tune the broadcast band will vary slightly depending on the length of the antenna you are using. This is because of the simplicity of the design where the antenna and the variometer coils form the tuned circuit. Leave plenty of lead length at the start and end of each winding to allow for the flexible connections.  A coat of varnish will help hold all the windings in place.

Performance
    This set is no DX performer but is a simple design for close in domestic listening, selectivity is broad but local stations peak up nicely on 50 feet of antenna wire and a good water pipe earth. I have a 50KW national broadcast station about 15Km (9 miles) away and when tuned in, headphone volume is almost too loud. Tuning lower powered and more distant stations is possible but the 50KW is always there in the background.

Written and Photos by
John Hassell
VK6JAH

How a Variometer Works

    A variometer is a device that uses two coils to provide a continuously variable inductance. If two coils are connected in series and spaced apart so that their magnetic fields do not interact, then the total value of inductance is simply the sum of the inductance of each coil.

    If the coils are placed in a position where their magnetic fields can interact then interesting things happen. The value of the total inductance is found to be either larger or smaller than the sum of the individual coil inductances, depending on the positioning of the coils and whether the wire turns are in the same or opposite directions. (in-phase or anti-phase). The amount of inductance that is gained or lost is due to a phenomenon called Mutual Inductance.

 
    If the coils are placed in-phase (the windings of both coils go in the same direction) then inductance is gained. If the coils are in anti-phase (the windings go in opposite directions) then inductance is lost

 
    Let’s check out the spiderweb coils used in the variometer crystal set. Using the formers as described and winding on 40 turns of wire gives a coil measuring about 110uH on my inductance bridge.

 
    Connecting them in series, with the end of the first coil connected to the beginning of the second coil and the coils well separated, total inductance is as expected around 220uH.

 
    If the coils are now carefully laid one on the other in-phase (both windings in the same direction) the total inductance increases to about 400uH. That’s nearly twice the value of inductance we would expect from just adding the inductance values of both coils.

    On moving the coils apart the total inductance value will fall back to 220uH. In fact any inductance between 220uH and 400uH and be produced by careful positioning of the coils.

     By reversing the connections to one coil (end of first coil to end of second coil) or by just flipping the coil over puts it in anti-phase (windings in opposite directions). When one coil is laid on the other a decrease in total inductance to around 50uH is seen. Again, moving the coils apart will produce any value between 50uH and 220uH.

 
    So here we have a way of smoothly tuning our crystal set by moving coils rather than using an expensive variable capacitor. But there is a problem. If you have built the variometer crystal set you will have found that the change in inductance is not enough to tune across the whole of the medium wave broadcast band. The way around this is to tune the bottom of the band with coils in-phase and switch to an anti-phase connection to tune the top of the band.

 
John Hassell
VK6JAH