The coil formers for the bottles are made to be a sliding fit. One way to do this is to roll each bottle in a couple of thicknesses of paper, cover that with a layer of thin plastic sheet, such as is used for food wrapping, and then wind on two or three layers of glass cloth as shown in Fig. 15.

The glass cloth is impregnated with epoxy resin and left to cure. The final thickness of the former wall should be about 5/64 in. Once the epoxy has hardened, the outer surfaces should be sanded smooth, and two wooden cheek pieces epoxied on. This gives a very rugged former. Two small holes are drilled in one cheek, one close to the bottom, the other near the top to bring the ends of the coil out through. The former is then covered with a layer of insulating tape.

WINDING THE DETECTOR COILS

Carefully solder an eight inch length of fine plastic covered multistrand wire to the start of the coil wire and insulate the joint with thin sleeving. Three inches of the multistrand wire are passed out through the bottom hole in the cheek to make the wiring connections to. Seven hundred and fifty turns of 24 s.w.g. enamelled coil wire are now wound on The winding should be as neat and even as possible.

If a lot of rough use is foreseen in the future, it might be prudent to spend a little more and use coil wire with a tougher coating. A suitable coating is armoured polythermaleze. With the winding completed the free end is joined to another piece of multistrand, the joint insulated and the wire taken out through the remaining hole in the cheek.

The complete winding is covered with a couple of layers of plastic insulating tape. For further protection put on a layer of thin foam rubber or plastic. At this point take a break to steady
the hands and then make the second coil in exactly the same way. Now if it is possible to measure the coils, the inductance of each should be 0.021H and the resistance about 7.9 ohms.