MEASURING LENGTH
Most of the time, this is a straightforward problem. A straight ruler or meter stick is aligned with the length segment to be measured and only care in matching the segment's boundary points to ruled marks (estimating between marks) is necessary.
VERNIERS AND MICROMETERS
Greater precision, if the distances are not more than a few centimeters, is afforded by a caliper (see Fig. 1), which can be set to match the distance in question. The distance is in effect memorized by the instrument, which can then be removed to an area where it can be read with ease. The caliper method has a very large application in length measurements. The vernier caliper in Fig. 1 can be used to measure both inside and outside diameters, and even depths, by using the different sets of jaws or the probe at the end.
(a)
3cm on the fixed caliper 0.22mm on the vernier scale
0 1 2 10
3 4
3mm on the fixed caliper
Total measurement:
30.00+3.00+0.22=33.22mm
(b)
Fig. 1
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A vernier scale (see Fig. 1b) provides a better method of reading between the ruled lines of a main scale than simply estimating by eye. To read a vernier scale, first note how many divisions on the vernier are equal to an integral number of main scale divisions. In the ordinary metric vernier, ten vernier divisions are equal to nine main scale divisions.
Hence the least count, or smallest value that can be read directly from a vernier scale of this type, is one-tenth of a division. The caliper divisions are millimeters, so that the least count would be one-tenth of a millimeter. The sketch in Fig. 1b shows the reading of a vernier.
Sleeve reads full mm: 11.00
Sleeve reads 1/2 mm: 0.50
Fig. 2
0 5 10
Total measurement:
40
Thimble reads: 0.368
35
11.00+0.50+0.368=11.868mm
Micrometer calipers (Fig. 2) are used to measure diameters quite precisely. Turning a handle moves a rod forward, by a screw thread, until the object to be measured is clamped very gently. In the usual metric instrument, one turn of the handle advances the rod a half-millimeter. A circular scale around the handle reads fiftieths of this halfmillimeter, or thousandths of a centimeter. One more figure is estimated between scale marks. Exactly the same kind of screw, rod, and scale are often incorporated into other measuring instruments.
Both vernier and micrometer calipers should always be tested for “zero readings” when fully closed (so of course should any other instrument).
MEASURING MASS
Several types of balances for weighing objects exist. With any kind, a “zero reading” (no load and no weights) should be taken first. Care should be taken not to overload any balance or to spill any corrosive materials on them. The trip balance, or pan balance is used for heavy objects, and for general weighing. The unknown goes in the left pan, the standard masses in the right pan. Beam balances are available in comparable size; there are also more sensitive ones for weighing small objects. In using a beam balance, the unknown is placed in the pan and a weight is arranged to slide along a calibrated beam.
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OHAUS TRIPLE BEAM BALANCE
The range of this general laboratory mass balance is increased over the simple single beam form by the use of several beams (see Fig. 3). One beam is for the largest mass increment -- the movable mass on this beam can be placed, in the balances used here, in five positions besides zero. Each corresponds to an increment of 100 gm. Another beam contains a weight that can be placed in any of ten non-zero positions corresponding to increments of 10 gm. In using these two beams, each movable weight must be placed definitely in the appropriate notch. The front beam contains a weight that can slide continuously along a marked scale to a maximum corresponding to 10 gm. The maximum mass that could be measured without additional features would be 500 + 100 +
10 gm = 610 gm. The least count of the scale is 0.1 gm, and the weight can be estimated to fractions of this smallest division.
Before measuring, a zero adjustment check is made with no mass on the pan, and all sliding beam masses at their left-most positions. A thumbscrew under the pan at the left is turned in either direction until the pointer indicates zero. Recheck the zero for each use of the balance.
A measurement is made by placing the mass to be measured on the pan, then moving the largest scale-mass (or ‘poise’) to the highest position that does not cause the pointer to change position. Then the second poise is similarly adjusted so the scale reads within ten grams less than the unknown. Finally the front poise is moved until the scale returns to the zero, or balanced, position. The unknown mass will have been determined to within the smallest subdivision of the front scale, a tenth of a gram.
Accurate to 0.1g, tiered beam scales read up to 610g
Fig. 3
100 g

10 g
500 g

100 g
10 g

0 .
1 g
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OTHER BALANCES
The chemical balance for weighing very small objects has two pans suspended below a light “see-saw” truss. Again the unknown goes on the left (except in the special technique of “double-weighing”). The standard masses for such balances should be handled only with forceps, lest perspiration etch them away. The surrounding glass case is closed while judging a result, so that drafts cannot cause errors. Correct balance is determined by watching the swing of a pointer, not by waiting for it to come to rest
(friction may hold it off center).
There are also electrically operated balances which give readings of mass directly upon dials, after prescribed settings have been made.
For most applications in our lab, the triple beam balance is adequate.
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