XXX INTRODUCTION. 



tude of the magnetic moment of that portion and its volume. The dimensional 

 formula is therefore the ratio of the formulae for magnetic moment and volume, or 



M.L<T-'P'^M'L-'T--P>. 



The conversion factor is therefore n^lr^-t~^p^. 



7. Magnetic Permeability,* or Specific Magnetic Inductive Capacity. 



— This is the analogue in magnetism to specific inductive capacity in electricity. 

 It is the ratio of the magnetic induction in the substance to the magnetic induc- 

 tion in the field which produces the magnetization, and therefore its dimensional 

 formula and conversion factor are unity. 



8. Magnetic Susceptibility. — This is the ratio of the numbers which repre- 

 sent the values of the intensity of magnetization produced and the intensity of the 

 magnetic field producing it. The dimensional formula is therefore the ratio of 

 the formulae for intensity of magnetization and magnetic field or 



or P. 



MJL-^T-ip-i 



The conversion factor is therefore/, and both the dimensional formula and con- 

 version factor are unity in the ordinary system. 



9. Current Strength. — A current of strength c flowing round a circle of 

 radius r produces a magnetic field at the centre of intensity 2TTCJr. The dimen- 

 sional formula is therefore the product of the formulae for magnetic field intensity 

 and length, or M-L*T~^P~*, which gives the conversion factor mH^t~'^p~^. 



10. Current Density, or Strength of Current at a Point. — This is the 

 ratio of the numbers for current strength and area. The dimensional formula 

 and the conversion factor are therefore M^L~*T~^P~* and m^l~H^^p~^. 



11. Quantity of Electricity. — This is the product of the numbers for cur- 

 rent and time. The dimensional formula is therefore M*L*T~^P~^ X T= M-L*P~*, 

 and the conversion factor m^fip~^. 



12. Electric Potential, or Electromotive Force. — As in the electrostatic 

 system, this is the ratio of the numbers for work and quantity of electricity. The 

 dimensional formula is therefore 



^L T~ — ^jy s-T^api 



aPiTp^" ' 



and the conversion factor m^l^t~'^p^. 



* Permeability, as ordinarily taken with the standard medium as unity, has the same dimension 

 formula and conversion factor as that which is here taken as magnetic inductive capacity. Hence 

 for ordinary transformations the conversion factor should be taken as i in the electromagnetic and 

 J~2/2 in the electrostatic systems. 



