With No. 26, 28, or 30 cotton-insulated wire make from fifteen to twenty wraps about the middle of the half-circular pieces of wood and carry the ends down through small holes in the base-block and thence through grooves cut at the under side of the block to the front corners, where they are to be made fast to binding-posts. A needle is to be set in the base-block midway between the two pieces of half-circular wood and through the strands of wire. Great care must be taken that the needle does not touch any bare wires, and as a precautionary measure it would be well to wrap the needle with a piece of insulating-tape where it passes through the strands of wire. Now place on the top of it a magnetized piece of steel, as described for the detector shown in Fig. 4. As it may not always be convenient to turn the instrument so that the needle points north, a small bar of magnetized steel or a stout needle that has been magnetized with a horseshoe magnet or a helix, may be laid across the half-circular wood pieces, so that it is parallel with the top layer of wires--in fact, it should rest on top of them. By means of this needle, or bar, the magnetic piece of steel balanced on the vertical needle between the upper and lower strands of insulated wire may be held in one position no matter which way the block is turned. When the current passes in through one binding-post and out through the other (having thus travelled through the coil on the half-circular blocks) the needle is deflected and points out at the brass bar and back at the upright block. When making any of these pieces of apparatus, where delicately balanced magnetic needles are employed, all parts of the mounting blocks or other sections must be put together with glue and brass nails or screws. It will not do to use steel or iron nails, screw-eyes, or washers, nor pieces of sheet-iron, tin, or steel, for they will exert their influence on the vital parts of the apparatus and so destroy their usefulness. This is not so important when making buzzers, bells, motor-induction coils, or similar things, but in delicate instruments, where magnetic needles or electro-magnets are used for recording, measuring, or detecting, iron and steel parts should be carefully avoided, except where their use is expressly indicated. An Astatic Current-detector Astatic current-detectors and galvanometers are those having two magnetic needles arranged with the poles in opposed directions. The ordinary magnetic or compass needle points to the North, and in order to deflect it a strong magnetic field must be created near it. For strong electric currents the ordinary single-needle current-detector meets all requirements, but for weak currents it will be necessary to arrange a pair of needles, one above the other, with their poles in opposite directions, and placed within or near one or two coils of fine wire. This apparatus will be affected by the weakest of currents, and will indicate their presence unerringly.