Robert Gibson Griswold

The jolly balance is especially convenient in the determination of the specific gravity of solid substances, insoluble in water, on account of the rapidity with which the work can be done. Owing to the fact that it is not necessary to obtain the actual weight in air, and that the readings are merely comparative, skill in rapid handling and accuracy in readings is soon acquired.

The specific gravity of such bodies as pieces of metal, alloys, ores and most minerals in general can be determined in a few minutes. The base a, is of triangular form and provided with three leveling screws, as shown, to level the instrument and bring the pans c with their suspension into a vertical line parallel to the pillar d. The triangular base a, is best made of 1 1/4" oak, as this adds sufficient weight to make the instrument steady while in use. It is sometimes advisable to attach to the under side of the rear base arm a piece of sheet lead, say 1/4" thick so that overturning will not be likely. Over the centre of the base glue the circular block c, and through both the base and block a 7/8" square hole is cut, into which the pillar d is driven and glued.

The pillar d is best made of maple, 1" square. The lower end is tenoned to fit into the 7/8" square hole in the base. The shelf f, is circular and 4" in diameter, being glued to the bracket g, the latter in turn being secured by two thin screws to the pillar as shown. Upon this shelf is placed the beaker of water during the determination.

To the top of the pillar is fastened a brass guide h, being provided with a 1/2" hole through which a 1/2" brass tube passes. Just below on the same pillar is fastened another guide i, having the same size hole. These two guides carry the rod or tube j. The tube j, is the only moving part of the balance and carries the spring k suspended from an arm 7, together with the pans c-c. It is made of 1/2" outside diameter brass tubing which is quite true to size and very straight. The upper end is fitted with a plug which is sweated in, the end being filed off perfectly square. This plug is then drilled and tapped for a 10-32 brass round head screw. The arm l is filed out of 1/16" hard sheet brass and provided at the outer end with a boss and screw by means of which the spring k readily attached or detached.

To the lower end of the tube j, is attached a piece of thin sheet brass which just bears against the back of the pillar and keeps j, from turning. The pressure should be only sufficient to accomplish this object, and no move else the tube will work stiffly and accurate adjustment be rendered impossible. The construction of the lower end of this rod is shown in Fig. '3, also the lower guide and the stud to which the operating wire is attached. The spriug is simply soldered to the end of the tube.

The tribe j, is raised and lowered by means of a fine steel wire passing over a sheave on the top of the pillar and wound around a drum at the base, this drum being provided with a hand-wheel to enable it to be turned in either direction. The shaft passes through a hole in the pillar and from the hole is cut a saw slot running longitudinally for 3/4" on each side. A screw passes from the back into the front of the pillar by means of which the wood may be compressed upon the shaft and thus holding the drum from turning. The wire should be about No. 30 or 32 steel.

Make the pans c-c, from sheet aluminum. They may be cupped if desired by beating to shape with a ball-pene hammer on a block of hard wood afterwards trimming to exact shape. They are supported by three fine copper or brass wires as shown. Pass the wire through the hole in the upper pan, then wrap once around itself as shown and then pass down to the lower pan. The intersection of the three wires from the upper pan is joined by a drop of solder to a straight piece of brass wire, about No. 25. One inch and a half above the joining of the three wires and this straight brass wire solder a small brass washer 3/8" in diameter, and placed at right angles to the wire, to prevent undue oscillations.

To the pillar attach a block o, provided with two brass forks as shown in Fig. 4, which limit the travel of the pans. The spring k, is made of fine steel wire, No. 30 B. & S. wound on a slightly tapering wood mandrel, 12" long, 3/4" in diameter at one end and 1/2" diameter at the other. The coil should be about 5" long when closed. Solder the upper end, which is the end with the smaller diameter, to a short piece of copper wire to afford a means of attachment to the arm The lower end should be made into a small hook to which the pans are attached. The reason for making the spring tapering is to avoid the undue stretching of the upper coils due to the weight of the coil itself. With this method of construction the coils gradually increase in strength from the bottom upwards.

Just below the block o, secure to the pillar a piece of mirror p, as shown in Fig. 4. Across the face of the mirror scratch a fine line with a sharp end of a file, at right angles to the vertical axis of the instrument. Blacken the supporting wire of the pans as indicated with india ink or lamp-black and oil, having the upper end of this blackened portion level with the line on the mirror when the washer is midway between the forks. Make a millimeter scale r of Bristol board about half a meter in length and attach to the side of the pillar as shown, numbering from the bottom upwards by tens. To the lower end of the rod j, fasten a brass pointer s, the sharpened end of which will just clear the surface of the scale. Make the scale divisions distinct and the ends of the pointer very sharp.

To illustrate the use of the balance, a determination of the specific gravity of brass will be used. The beater is nearly filled with freshly boiled distilled water. The rod j, is then adjusted by means of the hand-wheel until the edge of the blackened portion of the wire coincides exactly with its image in the mirror when looked at from the front. The reading on the scale is then taken and set down.

Now place the piece of brass in the upper pan and again adjust the rod j, until the image and blackened portion coincide and set down the reading, subtracting the first reading from it, the result corresponding to its weight in air. Then place the piece in the lower pan, being sure that it is completely covered with water, and adjust as before. This reading minus the first corresponds to its weight in water. Then calculate the specific gravity as follows :

Reading with body in air 127.3 mm.

Reading with pans empty 7.4 mm.

Spring extension due to weight in air 119.9 mm.

Reading with body in water 113.1 mm.

Reading with pans empty 7.4 mm.

Spring extension due to weight in water 105.7 mm.

Weight in air 119.9 119.9

Wt. in air - Wt. in water = 119.9 - 105.7 = 14.2 =

8.4 = specific gravity of the brass.

From the above will be seen that this method agrees precisely with the method in which a balance and weights are used. The above form of calculation is given in order that a clear understanding of the principle involved may be had by those unfamiliar with this class of work. It will often be noticed that text books give the following formula which is identically the same thing,

B-A/B-C= Specific gravity

A = Instrument reading, nothing in either pan B = Reading with substance in upper pan C = Reading with substance in lower pan

Inserting the assumed values given above we have

A = 7.4 mm. B = 127.3 mm. C = 113.1 mm.

127.3 - 7.4 119.9

127.3 -113.1 = 14.2 = 8.4 = specific gravity