Physics: Brief summary on measurement

Until the end of the eighteenth century, the study of material things was treated as a single aspect of human thought and called natural philosophy.

All measurements in Physics, even of such things as electrical current, are related to the three chosen fundamental quantities of length, mass and time. In 1960, the general conference of weights and measures recommended that everyone should use a metric system of measurement called the international system of units. The SI units are derived from the earlier MKS system, so called because its first three basic units are the metre, the kilogram, and the second.

The SI unit of length is the metre, originally defined as the distance, at 0 Celsius, between two lines on a platinum-iridium bar kept at the international office of weights and measures at Sevres near Paris. In 1983 the general conference of weights and measures redefined the metre as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second. For most practical purposes we still have to use metal standards which are checked by an interference comparator and this uses the wavelength of light. Various other metric units of length are related to the metre by either multiples of submultiples of 10. Thus, 1 kilometre=1000 metres, 1 metre=100 centimetres, 1 centimetre=10 millimetres. Very small lengths are measured in micrometres and nanometres, 1 metre=1 000 000 micrometre or 1 000 000 000 nanometre.

For measuring the diameter of a piece of wire and similar small distances, a micrometer screw gauge is used.

The mass of a body is the quantity of matter it contains, and the basic SI unit of mass is the kilogram. The standard kilogram is the mass of a certain cylindrical piece of platinum-iridium alloy kept at Sevres. Its various multiples and submultiples are: 1 tonne=1000 kilogram, 1 kilogram=1000 grams, 1 gram=1000 milligrams, and 1 gram=1 000 000 micrograms. The weight of a body is the force it exerts on anything which freely supports it and normally, it exerts this force owing to the fact that it is itself being attracted towards the earth by the force of gravity. The unit of weight which is a force is the newton. An important distinction between mass and weight is that the mass of a body does not depend on where the body happens to be, whereas the weight of a body can vary from place to place.

The volume of a liquid is measured in litres. The litre is 1000 cubic centimetres and, when the standard platinum-iridium kilogram was constricted in 1889, it was intended to be the mass of 1 litre of pure water at the temperature of its maximum density, 4 degrees Celsius. The litre was then officially defined as the volume of 1 kilogram of pure water at 4 degree Celsius. 1 Litre=1000.028 centimetre cube. At the time it was decided to leave the matter as it stood and to divide the litre in 1000 equal parts called millilitres so that, 1 cubic centimetre= 0.999 972 ml. In 1964, the general conference of weights and measures redefined the litre as equal to 1000 cubic centimetre. This means that the litre is, by its new definition, related directly to the metre and not the kilogram. The measuring cylinder is for measuring flask and the pipette for getting fixed pre-determined volumes. The burette delivers any required volume up to its total capacity, usually 50 cubic centimetre, and is long and thin to increase its sensitivity. Burette divisions generally represent 0.1 cubic centimetre, but measuring cylinders may be graduated at 1, 5 or 10 cubic centimetre intervals according to size. Readings on all these instruments are always taken at the level of the bottom of the meniscus or curved surface of the liquid. Mercury is an exception, as its meniscus curves downwards. Care should betaken to place the eye correctly so as to avoid parallax errors. When taking readings, the pipette and burette must be upright and the cylinder and flask must stand on a horizontal bench, otherwise errors may arise from titling.