Measurement of Volume

For measuring the volume of a liquid, different graduat­ed glass vessels are available, the choice of which depends on the circumstances. Volumes are measured in cubic centi­metres (cu. cm.) or millilitres (ml). The litre is equal to 1000 cu.cm.

The unit of volume for all scientific purposes is the vol­ume of a cube each edge of which is one centimetre in length. This unit is called the cubic centimetre, and is generally written cu.cm. or cm³.

The unit of volume in the metric system is the litre, which is the volume of a kilogramme of pure water at the temper-

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ature of its maximum density (4^oC). The litre is thus for all practical purposes equal to 1000 cubic decimetres.

Different measuring vessels are used in the laborato­ries. They are: the measuring cylinder, flask, pipette, burette and others. The measuring cylinder is for measuring or pouring out various volumes of liquid; the measuring flask and pipette for obtaining fixed, pre-chosen volumes. The burette delivers any required volume up to its total capacity, usually 550 cu.cm., and is long and thin to increase its sen­sitivity. The divisions may represent 0.1 cu.cm., while in the case of the measuring cylinder they may represent 1.5 or 10 cu. cm. according to the size of the cylinder.

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Time

Time as mass and length are fundamental physical quan­tities. We all realize instinctively what is meant by this word. The earth moving in its orbit round the sun and daily rotating on its axis forms a reliable natural "clock" which we can use to provide a satisfactory unit of time. To us on earth it is the sun which appears to move. The time interval which, on the average, separates the two successive moments when the sun is highest in the sky is called the mean solar day. Exactly one twenty-fourth of this is the hour, one sixtieth of the hour is the minute, and one sixtieth of the minute is the mean solar second. The mean solar second is the funda­mental unit of time in both the British system and the M.K.S. system. Time is measured by clocks of various kinds. Mechanical clocks are, however, a comparatively recent invention. The latest scientific clocks are electrical.

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Text. The metric system

During the last decade of the eighteenth century, the French Government made an entirely new standard of length — the metre.This was intended to be 1/40,000,000 of the cir­cumference of the earth measured through the poles.Тhe present standard, a bar of a platinum-indium alloy which is very resistant to the corrosion in the earth's atmosphere. was set up in 1875. Later, more precise measurements have

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handle the standardization ... metric units, and copies ... the metre were distributed ... thirty-one nations. 3, The, metric system was designed ... simplicity, and other units used are obtained ...the metre multiplying or dividing ... tens.

5. Make up and write 5 questions to these sentences:

1. The metre was originally defined as one ten-millionth part of the distance from the Equator to the North Pole, meas­ured along the meridian through Paris. 2. The distance between two fine lines scratched on a particular metal bar was supposed to equal this length, but the measurement was done inaccurately.

6. Read and describe calipers with the help of the figure. Describe how you would try to measure as accurately as possible the diameter of a ball.