A practical magnetic detector may be made quite simply as follows:

A suitable baseboard for the instrument is first selected from straight-grained pine, 18 inches long, 6 inches wide, and 7/8 inch thick.

Procure the works from an ordinary clock, preferably of the eight-day variety, although those from an ordinary alarm clock will be chosen here for the sake of simplicity. Remove the balance wheel and all unnecessary gears, screws, etc. To one end of the spindle of the last cogwheel solder a narrow strip of tin 1 inch long and 1/8 inch in width, to serve as a dog to hold a wind-brake, this to cause the wheels to revolve slowly and quietly. The tin strip should have a small hole punched through the center and placed over the end of the spindle, which projects a trifle from the under frame. A small drop of solder will secure it, after which any form of small cloth or paper vane may be attached by a wire loop or frame. Owing to the difference in construction of various clockworks, it is difficult to specify any shape or position of the brake, but the one shown in Fig. 240 gives the general idea. Cloth over a frame is preferable to paper or cardboard, as it moves silently. Allowance should be made for the movement of the vane, cither by cutting away the wood around it, or projecting the vane through a hole in the base, and supporting the whole instrument on a superficial base by means of cleats. The spindle to which the hands are attached serves for the driving shaft, and should be soldered to the cogwheel through which it passes, as ordinarily it is held by the friction of a spring pressing against it.

Two wooden disks, preferably birch, are now cut out 4 inches in diameter and 3/8 inch thick. Upon the periphery of each disk-is cut a groove of the shape shown in Fig. 241.

From a piece of heavy sheet brass cut a square 2 by 2 inches and drill a 1/8-inch hole in each corner and one in the center to tit the driving spindle on the clockwork. Place in position on the spindle and fasten with solder, being careful to keep it true. Hollow out the center of one of the wooden disks sufficiently to contain the lump of solder so formed, and fasten it to the brass square by means of small steel screws passed through the hole in each corner. A small magnetic screw driver will be found very useful for passing the screws into place through the open work of the clock frame.

Fig. 240 - The wind brake on the clock.

Fig. 241 - Details of the pulleys.

The clockwork is now mounted on one end of the board, the center of the disk being 3 inches from the edge. Stove bolts passed through open parts in the frame from the bottom of the baseboard and fitted with nuts and washers will be found the best method of doing this. A hole should be bored in the base-board immediately beneath the winding stem, to allow for the insertion of the key. Next cut a block of soft wood 5 inches square and of a thickness of 1/16 inch less than the distance between the top of the baseboard and the under side of the mounted disk. The remaining disk is now fitted with a brass bushing and a 1-inch round-head brass screw selected to fit the hole in the bushing nicely, and passed through it into the block of wood just mentioned, placing a washer beneath the disk and one under the screw head (Fig. 241). Fasten the block to the baseboard in a position so that the distance between centers of the disk shall be 12 inches.

This finishes the framework, and the coils should now be wound and adjusted. Obtain a piece of annealed glass tubing, as thin as possible, 2 inches long and 1/4 inch external diameter. Hold the ends in a Bunsen flame just long enough to smooth the rough portions, flaring one end slightly with a small stick of wood. This prevents chafing of the iron rope.

Fig. 242 - the primary and secondary coils.

In winding the primary coil over this tube it is a good plan to tie the ends tightly with thread, to prevent slipping. The wire used should be No. 36 silk-covered, and should measure 10 feet in length. It is wound in a single layer as closely and evenly as possible, leaving 6 inches of the wire at each end for con-necting. The coil when wound should occupy a space of 1 1/2 inches in the center of the tube. Give the whole a good coat of shellac and allow to dry.

Over the coil and tube so formed are slipped two small disks of 1/4-inch soft wood 1 1/2 inches in diameter (Fig. 242). The hole in the center of the disks should be just large enough to fit over the coil tightly, and shellac used to hold them in place. They should occupy a position in the center of the tube, being set 3/8 inch apart. When they have become firmly fastened in place the space between them is wound full of No. 36 silk-covered wire, leaving free ends about a foot long for connecting.

Tube and coils are now placed in position on the baseboard so that the interior of the tube is in line with the grooves on the periphery of the disks, and the coils midway between them (Fig. 243). Support the tube on a pair of blocks, as shown, using a liberal amount of shellac to hold it in place.

Cut out another wooden block 4 inches long, 2 inches wide, and of about the same height as those supporting the tube. Fix this block lengthwise in the center of the baseboard. Procure a small permanent magnet of the horseshoe variety, and mount it on the block in such a position that its north pole will be pointing directly in front of and nearly touching the outside turns of the secondary coil (Fig. 243), while its south pole will be opposite one end of the tube. If the disk on the clockwork revolves from right to left (as it ought), the south pole should be to the left of the center of the tube and coils; if in the opposite direction, to the right. It is immaterial which pole is in front of the secondary coil, as long as the remaining pole is in the proper relation to the direction of the moving band, about to be described. The commercial instrument is fitted with two magnets, like poles adjoining, and facing the center of the secondary coil, but the difference in effectiveness of this arrangement is so slight as to be unnoticeable.

Fig. 243 - The detector assembled on the base board.

Fig. 244 - Winding the wire band.

We now come to the last, and if not properly made, the most difficult and exasperating part of the detector, the moving hand or rope of iron wire. To the uninitiated this has always been a source of great difficulty and annoyance, and though simplicity itself when made in the following manner, attempts at other methods are almost sure to result in a bungling tangled mass of stray loops and ends.

The wire of which the hand is made is No. 36 silk-covered, iron wire. Select a soft pine board 7/8 inch thick about 3 feet long and 4 inches or 5 inches wide. Drive two nails to a depth of 1/2 inch in the board at a distance apart equaling twice the circumference of the oval formed by the two wooden disks, when measured by a string passed around the grooves. Starting at one nail (Fig. 244) wind the wire from one to the other, always winding in one direction: that is, so as to inclose the two nails in a narrow coil of wire. When the total number of strands equals 100 the ends are connected, and one nail is cautiously withdrawn from the board, keeping the wire still on it, and drawn taut (Fig. 245). Twist the strands into a rope, keeping them taut, and remove the remaining nail from the board. Both nails are now removed from the ends of the band, being careful not to disturb the loops formed by them. Thread the band through the glass tube, passing it around both pulleys and bringing the ends together between them. The two ends are linked together by threading a separate piece of the iron wire through and through them (Fig. 246), drawing tight after each threading, and connecting the ends of the wire by tying or twisting, as in the case of the band.

Fig. 245 - Removing the wire strands from the board.

Fig. 246 - Method of connecting the ends.

This completes the working parts of the detector, and any casing may be fitted to it and finished according to the ideas of the operator.

A good casing is made by fitting the sides and ends with 3/8-inch hardwood strips extending 1/2 inch above the surface of the disks. This forms a box with a top open, and a nice-looking instrument is made by attaching a glass door by hinges to cover it and/protect the working parts from dust and injury (big. 247).

Fig. 247 - General view of the magnetic detector.

The ends of the primary coil are brought to binding posts in the side of the box nearest them, and those of the secondary connected to another pair of binding posts, one on each side of the first two. If desired, a false bottom of pressboard can be fitted beneath the disks, leaving only the coils and tube, magnet, band, and disks visible.

It will be noticed in the case herein cited that the winding stem is situated in the base of the instrument - a great inconvenience that can be remedied only by gears or ratchets; but this is hardly worth while, in view of the great advantage to be gained by using an eight-day clock, which, in addition to its ability for long running, usually has the winding stem on its face. The proper speed of the driving disk is that which will cause the moving band to complete the circuit through the tube in about two minutes.

Aerial and ground are connected to the terminals of the primary coil, and the telephone to those of the secondary. An almost inaudible hissing sound, in the telephone, as the band slowly threads its way through the tube and around the pulleys, shows the detector to be in working order.