RESISTANCE OF MATERIALS 83 



63. System of Units. The fundamental units of mechanics 

 are those of time, length and mass. 



The unit of time is the second of which there are 60 per minute, 

 and there are 60 minutes in an hour, the hour being the -^ part of 

 the average solar day. 



The unit of length is the centimetre (see 18). The practical 

 unit is the metre = 100 centimetres and the multiples are the 

 kilometre (1,000 metres and the nwriametre 10,000 metres). 

 The English mile (1,609'315 metres) and the knot or nautical mile 

 (1,855 metres) are used in England and America. 



The unit of mass is the gramme ( 18), and as multiples are used 

 the kilogramme (1,000 grammes) the metric ton (1,000 kilo- 

 grammes), etc. 



The centimetre-gramme-second system is called the C.G.S. 

 system, and dates from the international congress of electricians 

 in 1881. 



In this C.G.S. system, the unit of force is the force which gives 

 the mass of the gramme an acceleration of 1 centimetre. It is 

 called the dyne. As the acceleration of gravity, at Paris, is 

 approximately 9 - Sl metres = 981 centimetres, the dyne is there- 

 fore smaller than the weight of 1 gramme and is -g^ T gramme or 

 1 gramme = 981 dynes. 



The unit of work or erg is the work of a dyne through a 

 distance of 1 centimetre. As the kilogramme = 981,000 dynes 

 and the metre = 100 centimetres, it will be found that the kilo- 

 grammetre, the practical unit, equals : 



981,000 X 100 = 981 x 10 5 ergs. 



Use is also made of a unit which equals 10 7 ergs : the joule which 

 corresponds consequently to : 



10 7 1 



- = -1019 kgm. approximately -102 kgm. 



981 X 10 5 9-1 



The power of a motor is the number of units of work done per 

 second. In practice, the name of watt is given to the unit cf 

 power which gives 1 joule per second, or OT02 kgm. per seccnd. 

 The industrial units such as the poncelet = 100 kilogrammetres, 

 the horse-power = 75 kilogrammetres is equal to : 



= 981 watts and -: = 735'75 watts respectively. 

 '10.2 ' L(JA 



As to the equivalence between mechanical and thermal energy, 

 experiment has shown that 426 '4 kgm. exactly are transformed 

 into one grand calorie (C) which is the quantity of heat able to 

 raise a kilogramme of water frcm to 1 degree centigrade. 



