Jolly Balance Instructions

The parts

There are five screws / knobs you need to use. Ignoring any of the small vertical ones, there are five horizontal knurled screws/knobs, four on the left and one on the right. The top one, "A" is a set screw which, when loosened, allows the top platform to which the spring attaches to move freely. The next one is the large knob on the left, "B"; this is a geared knob that moves the whole top section of the balance vertically when turned. The third screw, "C", is a set screw that keeps immobile the platform to which the alignment mirror is attached. The fourth screw, "D" holds the beaker platform in place. Finally, the screw on the bottom-right, "E", is a set screw that keeps the marked scale tight.

Instructions

  1. Set up the instrument.
    1. Select a spring of appropriate strength for the weight of the specimen you wish to test.
    2. Screw A should be loose, B should be turned so that the top assembly is as low as it goes, C and D should be tight, and E should be loose. The scale should read zero on both sides.
    3. The spring should be attached to the top platform, then the black wire with the leveling disk is attached, and finally the double-pan assembly is attached to the hook at the bottom of the wire.
    4. Be sure screw A is loose. Holding knob B tight, raise the top platform (that the top of the spring is attached to) until the leveling disk is near the line in the mirror. Tighten screw A.
    5. Adjust the platform that the beaker sits on (loosen screw D) and the water level in the beaker so that the bottom pan is submerged along with most of the wires that attach it to the top pan, and the top pan is dry (it should not touch the water at any time).
    6. Adjust the mirror platform (loosen screw C) so that the line is a bit above the reference disk on the black wire.
    7. Level the instrument by adjusting the two screws on the rear support legs so that the black wire goes through the middle of the hole in the mirror platform.
  2. Zero the instrument
    1. Loosen screw A. Holding knob B tight, raise the top platform slowly until the leveling disk is exactly aligned with the line in the mirror. Tighten screw A. (You can perform fine adjustment of the mirror with the small vertical screw on its top).
    2. Note that now the spring is extended without any sample, the disk is at the line, and the scale reads zero on both sides. Any extension due to the weight of the sample can now be read, with zero as the pre-sample reference. Good!
  1. Measure the spring extension with the sample in air.
    1. Holding the black wire, place the specimen in the top pan and turn knob B until the spring holds the weight of the specimen. (If you turn B as far as it goes and the spring never holds the weight of the specimen, then either try a smaller specimen, or start over with a stiffer spring).
    2. Adjust B so that the disk is exactly aligned with the line in the mirror.
    3. Tighten screw E.
    4. Note that the scale on the left now represents the additional extension of the spring with the sample in air (call this EA), and the scale on the right reads zero. (If EA is small, then you should start over with a looser spring).
      1. You need to read the vernier scale to get the last decimal point of the measurement. To do this, you read the first decimal at the zero of the vernier, and the second is given by which vernier tick mark aligns with a mark on the scale itself. In the image at right, the left scale reads 2.72 and the right scale reads 1.63.
  1. Measure the spring extension with the sample in water.
    1. Holding the black wire, place the specimen in the bottom pan.
    2. Adjust B so that the disk is exactly aligned with the line in the mirror.
    3. Note that the scale on the right now represents the difference in extension between the sample in air and the sample in water. This is EA-EW (not EW!). See image at left for the idea.

Calculation

Calculating the specific gravity is easy now. It is G = EA / (EA-EW).

-Dave Hirsch, 10/6/03