10 ELEMENTS OF LABORATORY WORK 



A length of 10 centimetres is called a decimetre. 



A length of 10 decimetres or 100 centimetres is called a 

 metre (equals 39*37 inches). 



A length of 1,000 metres or 100,000 centimetres is called 

 a kilometre. 



A length of -1 centimetre or '01 decimetre or -001 metre is 

 called a millimetre. 



The numerical value i.e. the number expressing the mag- 

 nitude of any quantity of matter or of any distance shows its 

 relation to a magnitude of the same kind selected as unit. 



Thus if M denotes a definite quantity of matter, and L a 

 definite distance, and m and I the respective units, then 



and j give the numerical values of these quantities. 



It is seen that numerical values vary directly as the quan- 

 tities, and inversely as the units with which the quantities are 

 compared. In other words, as the quantity increases the 

 numerical value increases, but as the unit increases the 

 numerical value diminishes. 



In practice the magnitudes of most quantities are expressed 

 by a number, followed by the name of the units used e.g. 

 20 centimetres, 3 grams. The necessity for the names arises from 

 the use of various systems of measurement. For example, 

 distances are, for other purposes, measured by inches, feet, 

 yards, miles, &c., and quantities of matter by ounces, pounds, 

 tons, &c. If one system were universal, numbers alone would 

 denote the magnitudes of physical quantities. We shall after- 

 wards see that all physical quantities, however complex in 

 their nature, may be measured by the use of three fundamental 

 units of length, mass, and time. This system is called the 

 centimetre-gram-second, or C.G.S. system. It will be seen 

 that all quantities are brought to a common scale by use of 

 this system. 



Having selected the centimetre as the unit of length, it is 

 necessary, in order to secure uniformity, to take as the unit 

 of area the quantity contained by a square, each side of which 

 is a centimetre ; and for unit of volume that quantity con- 

 tained by a cube, each side of which is a centimetre. A 



