John F. Atkins

Every amateur should possess an accurate galvanometer for general laboratory testing. While many high grade types of galvanometer are beyond the means of most electrical students, it is possible to construct an efficient instrument for a very small sum. Much of the -construction can be done with tools common to every workshop, but there are certain parts which require absolute allignment, and might work more perfectly if fitted by one of experience. A friendly jeweller or machinist could render valuable assistance in half an hour's time, should the amateur be without tools for this part of the work.

A Simple Galvanometer 217

First procure a cover to an ordinary, round, wood blueberry box. Cut out a circular piece of cardboard to fit inside the cover, for a dial. Wind as many turns of No. 32 cotton covered magnet wire around the outside edge as may be applied without crowding over the edge, say three layers of 12 turns per layer. Stick the wires together, and to the wood, with quick-drying shellac varnish.

Having found the exact centre of the cover, drill a small hole and insert a small piece of straight, hard, steel wire about 1-16" diameter and 1/2" long. The upper end of this rod should first be turned to a long, highly polished point. The amateur is cautioned to exercise care in fashioning this point, being sure there is no rough, feathery edge to it when completed.

Upon this point is to rest and rotate a pivot to be turned out of a piece of brass rod. The exact size and shape of the pivot is not imperative, but the shape, as shown in the illustration, should be followed quite closely.

A piece of straight ribbon steel, as long as the diameter of the cover, is next procured and cut and bent into shape as illustrated. The exact balance of the strip is next found, and a small hole punched to take the top of the pivot. Next solder the pivot and steel strip together and magnetize the steel by winding insulated wire around it and sending the current from two or three cells of battery through for a short time. Then place the needle in position upon the point and we have practically a compass, for it will be noticed that the needle pointis to the magnetic pole. By sending battery current through the wire coiled about the cover a deflection takes place; that is, the needle moves from north to another position and remains in that vicinity as long as the current is continued. A couple of binding posts, to the bases of which are at tached the ends of the coil and a mark or dot indicating north or zero on the dial, completes the galvanometer.

Now, as to its uses. In a simple galvanometer of this kind we find the needle susceptible to influences of any magnetic metal that may be brought near the instrument while testing. This interferes with accuracy, and care should be taken to see that this is avoided. With this instrument a comparison of current strength may be made, but its principal value is the measuring of resistance in connection with the