To every worker in physics or electricity a good and reliable galvanometer is a prime necessity; but the prices asked for such by instrument makers often constrain one to get along with some rude and imperfect makeshift. But at a merely trifling expense, an instrument may be made which shall be equal in performance to any that can be bought, and which requires but little mechanical skill on the part of the maker.
Procure 1 ft. of 3 in. brass tubing, 5 in. of 2 1/2 in. tubing, 6 discs of brass plate 3 in. diameter, and a piece of hardwood plank, or, better, vulcan'te, the latter to serve as a base to e finished instrument. From the 3 in. tube saw a piece 2 1/4 in. long and nicely square its ends. This is for the body or barrel of the galvanometer. Crosswise of this, and midway from either end, a slit 2 in. long and 1/8 in. wide is next to be made.
Now take the 2 1/2 in. tube, and with a broad half-round hie fit one end of it to the side of the barrel - a rather difficult feat for a novice. When fitted it is to be soldered in place, immediately over the slit in the barrel. In this and subsequent operations of soldering the joints are to be " sweat" together, that is, the pieces are bound in place with wire, plumbers' acid and solder put around the joint, and the whole heated in a lamp until the solder flows into the joint, when it may be "wiped" with a piece of cloth. Thus is formed the standard of the instrument, which nerves to support it upon its base. To this end a plug of wood may be driven firmly into the open end of the standard, and a large screw passed up through the base into it, thus binding the two together. The base mar be turned or finished in any form to suit the taste of the maker, and it should be provided with -3 levelling screws threaded through the base itself or through projecting arms of brass.
R, almnluium wire; A, mirror; N S, magnetic system; F, silk fibre; EE, dragon fly wings.
At the central point of the top of the barrel drill a small hole, and over the bole solder a brass ferrule for holding a glass tube, which lost is to carry the suspension arrangement. Now take your piece of 3 in. tubing again and saw from it two rings, each 3/4 in. wide. After smoothing the ends of these, slit them open and take out a small portion, so that they may just be sprung into the barrel. While in this position, with a little projecting, one of the discs is to be laid upon either ring and secured by soldering. Thus are formed two shallow caps for containing the coils. Through the centre of one of these cups make a hole 3/4 in. diameter, and also in each cup two fine holes, one near the circumference, the other near the centre, for passing out the terminals of the coils. In the enp having the large central hole, the small hole is to be made close by the edge of the large one.
The coils themselves may next be wound. Make a spool of wood, 1 in, between the heads, and having its core 3/6 in. diameter at one end, 5/8 in. at the other. The spool head on the smaller end of the core is made removable, so that the coil when finished may be drawn from the spool. Fin the spool to any convenient support with a large screw, and insert a peg near the margin of the free head, to serve as a handle for turning the spool in winding the coil. The wire to be used will depend upon the purpose for which the instrument is to be employed. No. 24 to SB wire is good for general purposes; but the general worker will find it advantageona to have three sets of coils of No, 16, 28, and 36 wire respectively, and it was that other caps and coils might be made at leisure that the extra tubing and discs were provided.
Before winding, the wire is to be cooked in hot paraffin until all air is driven off. Hake a smalt hole through the spool head close to the larger end of the core, pass one end of the wire through this hole, and then, guiding the wire with one hand and turning the spool with the other, fill up the spool, making the winding assnugandperfect as possible. To permit of adjustment, the outer diameter of the coil should be a trifle less than the diameter of the cup that is to contain it Carefully take away the removable spool head, and without disturbing the coil give it a thin covering of solid shellac upon its exposed face and edge. The shellac is melted and neatly smoobed upon the roil with a hot iron. The coil may now be most carefully removed from the spool, and its other face, as well as the portion within the conical hole, coated with shellac as above. The second and subsequent coils are made in the same manner. The coils are fixed in the cups by pouring melted rosin about them, first taking care to pass the terminals through the holes provided for them. - The needle or magnetic system next demands attention, and it will test the skill of the beginner.
A piece of No. 16 aluminium wire, 3 in. long, is flattened at either end for 1/2 in. of its length, and through one end a minute hole is pierced. A staff for carrying the magnets and mirror is so formed. For the magnets procure a rather wide watch spring, anneal it well, and file or grind a portion of it until it is made as thin as newspaper, about '07 mm. Cut from this 12 pieces, each 3/8 in. long, and roll them about a steel wire into little hollow cylinders 1/25 in. diameter. (Some manufacturers use short flat pieces of narrow watch spring for this purpose).
H, sliding wire lor adjusting needle ; F, silk fibre; N, glass tube.
The 12 cylinders are then to be dipped in a strong solution of potassium ferrocyanide, heated to bright redness, and suddenly plunged into cold mercury. By these means they are made extremely hard, and will retain a very strong magnetic charge. To magnetise, string them on a wire, and put in a solenoid through which the strongest available current, preferably that from a dynamo, is made to pass.
On little square scales of mica arrange the magnets in two sets of 6 each, taking care that in each set the poles of the individual magnets shall lie in the same direction. Secure them upon the mica scales with a very little shellac varnish, and in the same way the mica scales upon the staff, one at either end, being very careful that the combined poles point in opposite directions in the two sets of magnets. In front of the magnets near the upper end of the staff (the end having the minute hole) is placed a mirror, and fixed with shellac. These mirrors may be bought for a small sum of the dealers, or easily made by grinding very thin a piece of plate glass and silvering its unground'side. The ordinary microscopic cover glasses are rarely perfect enough to be used as mirrors. Our needle now needs only the addition of a pair of dragon-fly wings, in the position indicated in Fig. 108, to make it complete. These wings bring the needle quickly to rest after a displacement.
A glass tube 10 In. long is now to be fixed upright in the ferrule on the top of the barrel. A little sulphur melted upon the heated end of the tube accomplishes this. The top of the tube must be provided with an arrangement for suspending the needle. Fig. 109 shows how this is made. Another ferrule fits the glass tube. On it rests a small plate of sheet brass, which is perforated, and through the latter a split tube passes, grasping a wire, and moves in the tube with gentle friction. The ferrule, the plate, and the split tube are united with solder. To suspend the needle, remove the sliding wire and to its extremity attach with varnish one end of a long fibre of silk, such as may be drawn from white embroidery silk or a white silk ribbon (unspun silk fibre is preferable for this purpose, but the twisted fibre may be straightened by steaming). Press a little ball of wax upon the free end of the fibre, and drop the ball down through the split tube into the galvanometer barrel, and push the wire in place. The end of the fibre in the barrel can now be caught and threaded through the hole in the needle staff, secured, and the wings put through the slit at the bottom of the barrel, where they should swing freely in the tube below.
The coils can now be pushed into place, the coil having the large hole being the front one. In this hole a spectacle lens of 4 ft. focus, ground to a fit, is to be cemented. The suspension wire is moved up or down until the mirror is seen to occupy the centre of the coil. Two of the coil terminals are to be joined so that the current may circulate in the same direction in each coil, and the other two are connected to screw posts upon the base of the instrument.
Section of Galvanometer.
G, G, galvanometer barrel; M M, coils; A, mirror; B B, magnetic system; C, lens; T, tube for standard; N, glass tube; R, aluminium wire; X X, terminals of coils.
A small bar controlling magnet is provided, either upon a separate stand, or it may be attached to the glass tube with the aid of a split cork. The instrument itself is now complete, except some means for reading its indications. The following simple device accomplishes that purpose better than the most elaborate and costly telescope and scale. Procure one of those lenses sold as reading glasses. It should be about 3 in. diameter and 6 in. focus. " Make a stiff tube of paper 2 ft. long, 3 in. internal diameter. The tube should be furnished with a telescopic slide at one end, and in the slide a peep hole. The lens is to be fixed in the tube at its own focal distance from the peephole, and opposite the peephole, also in focus of the lens, a fine wire or spider line is stretched. Fig. 110 shows the device in section and will make the details clear.
A scale of equal parts printed or marked upon paper and attached to a strip of board is the only remaining detail. The telescopic device is secured so as to point directly at the galvanometer mirror, about 6 ft. distant, and a few trials will enable one to place the scale so that a distinct view of the divisions may be had upon looking through the telescope. Remember that the scale is seen reflected in the swinging mirror, and there will be but little difficulty in securing the correct adjustments.
An instrument made by the writer in the foregoing manner, though it has a resistance of only 50 ohms, gives a deflection of 20 divisions of its scale through a resistance of 250,000 ohms, the current being furnished by a single Darnell's cell. It can be made without a lathe. There is but a single screw about it, and the whole cost of construction need not be more than 10-12s.