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# Jolly's Balance

The apparatus consists of a long spiral spring carrying a pan into which weights or the object to be weighed can be put.

From this there hangs, by a fine thread, a second pan which is always kept immersed in water.

Behind the spring is a millimetre scale engraved on a strip of looking-glass, and just above the pan is a white bead, which can be seen directly reflected in the glass.

By placing the eye so that the top of the bead just appears to coincide with its own image, the division of the scale which is opposite to the top of the bead can be read with great accuracy.

(1) To weigh a small Body and find its Specific Gravity.

Place the object to be weighed in the upper pan, taking care that the lower pan is well below the surface of the water, and that the vessel in which the water is, is sufficiently large to allow the pan to hang clear of the sides.

Note the division of the scale which coincides with the top of the bead. Suppose it is 329.

Remove the object from the pan and replace it by weights until the bead occupies the same position as before. Let the weights be 7.963 grammes.

It may be impossible with given weights to cause the bead to come to exactly the same position.

Thus, we may find that 7.963 gms. causes it to stand at 330, while 7.964 gms. brings it to 327.5. The true weight lies between these two; and the addition of 0.001 gramme lowers the bead through 2.5 mm. We require the bead to be lowered from 330 to 329 - that is, through 1 mm. We must therefore add to our weight

The true weight then would be 7.9634 grammes.

The water should be adjusted so that its surface is above the point of junction of the three wires which carry the lower pan.

Next place the small object in the lower pan, and put weights into the upper till the bead again comes to the same point on the scale. Let the weights be 3.9782 grammes.

This is clearly the weight of the water displaced by the object, and its specific gravity referred to water at the temperature of the observation is therefore

To obtain the true specific gravity, we must multiply this by the specific gravity of the water at the temperature of the observation. Let this be 15°.

The specific gravity of water at 15° is 0.99917, so that the specific gravity of the solid is

(2) To determine the Specific Gravity of a Liquid.

Take a small solid which will not be acted on by the liquid, and place it in the upper pan. Note the point to which the bead is depressed, the lower pan being in water.

Now place the solid in the lower pan and put weights into the upper until the bead comes opposite the same mark. Let the weight be 3.596 grammes. This is the weight of the water displaced by the solid.

Remove the water and replace it by the liquid. Put the solid into the upper pan, and note the division opposite to which the bead stands. Let it be 263.

Put the solid into the lower pan, and put weights into the upper until the bead comes opposite to 263. Let the weight be 4.732 grammes. This is the weight of the liquid displaced by the solid.

Thus, the specific gravity of the liquid

This must be corrected for temperature as usual.

Experiments.

(1) Determine by means of Jolly's Balance the specific gravity of the given small crystal.
(2) Determine by means of Jolly's Balance the specific gravity of the given liquid.

Enter the results thus:

(1) Specific gravity of crystal.

 Scale reading with the crystal in the upper pan 329 mm. Weight required to bring the bead to same position 7.9634 gms. Weight required with crystal in lower pan 3.9782 gms. Temperature of water 15° C. Sp. gr. of crystal 2.000.

(2) Specific gravity of liquid.
 Scale reading with solid in upper pan, lower pan in water 329 mm. Weight required to bring the bead to the same reading with the solid in water 3.596 gms. Scale reading with the solid in the upper pan, lower pan in the liquid 263 mm. Weight required to bring the bead to the same reading with the solid in the liquid 4.732 gms. Temperature of the water 15° C. Specific gravity of liquid 1.315

Last Update: 2011-03-27