A Compact Galvanometer
A small portable galvanometer is one of the most useful instruments to the electrical experimenter. There are continually arising instances where it is necessary to test through and identify certain wires, for which purpose a small galvanometer and a dry cell are quite sufficient. For comparing the resistances by the well-known Wheatstone-bridge method, a galvanometer is, of course, indispensable. If the winding is made suitable, or by placing a shunt across the terminals to reduce the deflection, a small galvanometer will roughly indicate the current passing and thus enable one to compare his dry cells and eliminate the weak ones. Rough voltage comparisons may also be made by placing a resistance in series with the galvanometer.
For constructing this instrument, a good pocket compass, of about 2-in. diameter, must be procured. Prepare a neat little box with the four edges accurately beveled off. On the under side of this, carefully cut a channel, about ¹⁄₂ in. wide and 2¹⁄₂ in. long, to a depth that will bring the bottom of the slot within ¹⁄₈ in. of the top of the base block. Place two binding posts on the base, as indicated, and secure the compass in place with cement, or by two very small nails put through the bottom. If the glass cannot be removed, it will be necessary to solder the nail heads to the bottom of the compass box, after having carefully removed the lacquer.
The correct wiring will depend on the strength of the current handled. It is, however, very easy to get an idea of what the deflection will be under certain conditions by merely making a preliminary trial, after winding a fewturns of any magnet or bell wire at hand around a small piece of wood, and slipping the coil so formed into the slot on the under side of the base block. The winding may be from two or three turns of heavy wire up to several hundred turns of fine magnet wire, but after one or two trials, the maker will have no trouble in determining his particular requirements.
Galvanometer Made of a Compass Set on a Wood Base, with Coil and Wire Connections
Galvanometer Made of a Compass Set on a Wood Base, with Coil and Wire Connections
The final coil should be wound lengthwise on a wood core, and the whole packed neatly into the slot. Connect up the ends to the binding posts, and then glue in a thin piece to hold the coil in place.
By drilling a small horizontal hole through the base, as indicated by the two dotted lines in the top view of the working drawings, and inserting a small bar magnet, ¹⁄₈ in. in diameter, or less, the instrument may be rendered independent of the earth’s magnetism and used without reference to the north point. Such a controlling magnet reduces the time required to bring the needle to rest after it has been violently reflected.
¶Woodwork about a house, when primed with white lead made quite thin in raw linseed oil, will never blister unless moisture gets back of it. Yellow-ocher priming will cause blistering at any time up to 20 years.
¶Woodwork about a house, when primed with white lead made quite thin in raw linseed oil, will never blister unless moisture gets back of it. Yellow-ocher priming will cause blistering at any time up to 20 years.