Measures for Electric and Magnetic Quantities. 395 



§ 8. The Practical System of Measures as a Simple System. 



In the foregoing form the practical system has the incon- 

 venience that with every unit one must bear in mind the power 

 of 10 by which the general formula given in § 4 is to be mul- 

 tiplied. Nevertheless, as the Committee of the British Asso- 

 ciation has already remarked, by a suitable selection of the 

 fundamental units the practical can be made a simple system, 

 in which all the units can be represented merely by the for- 

 mulas given in § 4. For this purpose 1 . 10 -I1 ^r must be taken 

 as the unit of mass, and 1 . 10 9 cm or 1 . 10'ra as the unit of 

 length, while the time-unit remains 1 second. 



If the length 1 . 10 7 m be denoted by g to indicate that it is 

 equal to a quadrant of the meridian, and the mass 1 . 10 -11 ^r 

 by p, the practical units can be written as follows: — 



Weber [mj =pi q§ s~ l , 



Coulomb [e^\ =pi qi, 



Ampere [r rf ] =p2qis~ 1 , 



Yolt [EJ =piqis- 2 , 



Ohm [^] = S ,s ~ 1 j 



Farad [G d ]=q- 1 s 2 . 



This manner of representing them has manifestly the advan- 

 tage of greater simplicity ; and at the same time it is further 

 to be remarked that the units of other kinds of electric and 

 magnetic quantity, not contained in the preceding table, 

 assume a similar simple form when these fundamental units 

 are employed; while if gr, cm, s be used, the formula of every 

 new unit must be provided with a specially to be determined 

 power of 10 as a factor, by which the distinctness and ready 

 intelligibility of the formulas are impaired. 



The circumstance that, in order to obtain at once for elec- 

 tric units values agreeing to some extent with the quantities 

 we have at other times to measure, and hence convenient for 

 us, so very small a unit of mass and so very large a unit of 

 length must be employed, is to be regarded as a characteristic 

 peculiarity of electricity, and hence cannot be avoided. In 

 order, then, to facilitate the representation in investigations 

 in which such very small and very large units occur, it is 

 desirable to be able to state them briefly, not only by mathe- 

 matical symbols, but also in words ; and I would make a pro- 

 posal for that purpose. 



In the French system of weights and measures, as is well 

 known, for each kind of quantity, starting from the funda- 



