SOME USES OF MATHEMATICS 123 



to-day as a result of our greater knowledge of physical 

 magnitudes. A secondary standard was suggested by 

 Michelson, 1 the recipient in 1907 of the Nobel prize. 

 He suggested the wave length of light, choosing for that 

 purpose the red light emitted by cadmium vapor when 

 an electrical discharge passes through it. In terms 

 of this wave length he made a remarkably accurate 

 measurement of the meter, working, of course, with the 

 prototype at Paris. The distance represented by the 

 meter is, therefore, very precisely known to-day in 

 terms of an absolutely stable physical magnitude 

 which may be reproduced by any scientist La his labora- 

 tory. 



The remaining fundamental units from which the 

 scientist derives all the other units are those of mass 

 and tune. The unit for mass, or quantity of ponder- 

 able matter, is the gram which is the thousandth part 

 of the kilogram. The latter was originally intended 

 to be the mass of a cube of water, one tenth of a meter 

 on a side, at its greatest density. Although more 

 recent measurements have shown that it is not accu- 

 rately this mass, the kilogram is the accepted standard. 

 The unit of time, the second, has been discussed in 

 Chapter III. The second, the hundredth part of the 

 meter (1 cm.), and the thousandth part of the kilo- 

 gram (1 g.) are the fundamental units for scientific 

 purposes. 



In terms of these fundamental units the other units 

 are easily obtained when once the laws are known. Such 

 units are said to be derived units of the C. G. S. system. 



1 Cf. Michelson, "Light Waves and their Uses " Univ. of Chicago 

 Press, 1907. 



