Robert Gibson Griswold.

A pair of good balances is a very valuable acquisition to the amateur's dark room or laboratory, and they are often quite expensive. The one herein described is not very hard to build, is very sensitive, and compares very favorably with those being sold in the market for $15 to $20. In fact, about the only thing in a balance that does cost money is the skill neeessary to adjust them, and with ordinary patience, this can readily be done by an amateur so that the balance will render excellent service.

The beam a is cut from 1/8" sheet brass. Select a piece of sheet brass of the stated thickness, which is very flat and straight. Then lay out the shape of the beam thereon, and with the aid of a hack-saw and file, carefully cut out the beam and finish it as smoothly as possible with a dead smooth file, then crocus cloth and oil. Lay out a centre line passing through the centres of the end edges, and another at right-angles thereto at the centre. Then lay off the three holes for the knife edges so that the upper edge of the holes at the ends just touch the line and the bottom edge of the centre hole does the same. The holes at the ends are 1/16" and the central hole 1/8" in diameter. Take great care to drill them parallel.

At each end file a 1/16" slot forming a fork as shown in Fig. 2. This is to accommodate the pan hangers. The small adjusting screws at the ends are pieces of 1/16" brass wire threaded with a small die of about 60 threads per inch. One end is flattened and soldered to the back of the beam with a small bit of solder. The nuts are cut from a 1/4" brass rod and may be given a knurled edge by pressing on the edge with a mill file while rolling the nut on a hard block, provided no lathe and knurl are at hand. The end knife edges are made of 1/16" steel and the middle one of 1/8" round steel. Fig. 3 shows the general form of the edges and the end edges may be made by grinding a hollow on opposite sides so that the upper edge is about a 60° angle. The middle edge is filed down so that the edges on both ends are exactly in line. These knives are annealed before filing to rough shape, then hardened and ground on an oil stone to a keen edge. They must be placed in the beam so that all three edges are exactly in line; if not, slightly bend the beam by pening at the middle until they are so.

From a stout sewing needle make two hooks, as in Fig. 4, the hooks forming a right angle with each other. Anneal the needle first, and bend while still red, then harden by plunging into water. Polish the bearing surfaces with a stick and crushed rotten stone. The semi-circular groove in the end edges will prevent the hook from turning and striking the sides of the beam fork. The pointer or spear b may be made of 1/16" spring brass wire, or cut from a flat sheet and filed to shape, which really makes the best job. Fig. 5 shows the method of making the top, and the gravity bob screw serves a double purpose in holding it in place. A small groove should be filed in the beam to keep it always in the correct position. The spear must extend far enough beyond the knife edge to allow the supporting fork to clear.

The pans are made of 1/16" sheet aluminum, and with bows or hanging wires of either aluminum or German silver, riveted firmly to the pans from underneath as shown. The pans are 4" in diameter. In Fig. 6 is shown the supporting fork for the middle knife edge, and it should be filed out of a piece of tool steel that has been well annealed. Polish the bottom of the V well, that there may be little friction at this point. On the outside, as shown in faint lines, cement small pieces of glass to keep the edge from sliding too and fro. The knife must be just a little shorter than the distance between the end plates of glass, and should also be ground to a keen point, as shown in Fig. 3, to reduce the possible friction to a minimum. Mount this fork on a 3/196" brass rod long enough to pass through the base as shown.

The column is made of 5/8" tubing, and a flat piece soldered to each end, one to form a base and the other the top. Each is drilled to accommodate the rod which must be a very snug fit, but still free. To the base secure a piece of white celluloid, and mark twenty equal dimensions as shown. The base needs no description other than that it must be made of some hard wood, either walnut, mahogany or cherry. It is provided with two leveling screws and a stationary pin in the rear. Two ordinary brass wood screws are inserted directly under the middle of the pans to keep them from swinging. The heads should have thin pieces of felt glued to them, and by screwing them in or out, the spear may be made to point exactly to zero when at rest.

Under the base is the lifting mechanism, which is shown in greater detail in Fig. 7. The 1/8" rod passing in from the front terminates in a hook, and the latter is provided with a pin set at a 1/4" radius, which, by the rotation of the shaft, causes the rod carrying the beam support to rise and fall. To the bottom of this vertical rod is soldered a long, thin, brass spring which bears against the under side of the base, and keeps the beam from swinging from side to side, and to further secure it, a couple of brass nails may be driven into the base at one end. The outer end of the lifting shaft is provided with a knurled head 1 1/2" diameter, by means of which the beam is raised. File a very shallow groove in the brass spring directly under the vertical rod, into which the pin may slip and remain, to prevent turning while in use.

Now adjust the balance by means of the end screws until the spear stands at zero. Place any weight on the left pan and balance it exactly by placing small weights and finally sand on the right pan. Then transpose the weights, and if the pointer still swings to zero at rest the beam is correct. If not, the heavier weight will have been on the short arm, and this must be lengthened. To do this, lay the beam on a smooth piece of iron and lightly tap it with a flat face hammer. This will stretch it lightly. Now return it to the rest and balance again, repeating the above operation until an exact balance is secured upon a transposition of weights.

The sensibility of the balance is effected by the small gravity bob on top ; raising it increases the sensibility and lowering it decreases the sensibility. With careful work, and adjustment of the gravity bob, this balance should indicate a tenth of a grain with perfect distinctness under a load of say ten ounces. Polish all the metal forks and give a good coat of lacquer before making final adjustment.