INTRODUCTION. XXIX 



ELECTROMAGNETIC UNITS. 



As stated above, these units bear the same relation to unit quantity of magne- 

 tism that the electric units do to quantity of electricity. Thus, when inductive 

 capacity is suppressed, the dimensional formula for magnetic quantity on this sys- 

 tem is the same as that for electric quantity on the electrostatic system. All quan- 

 tities in this system which only differ from corresponding quantities defined above 

 by the substitution of magnetic for electric quantity may have their dimensional 

 formulae derived from those of the corresponding quantity by substituting P 

 forK. 



i. Magnetic Pole, or Quantity of Magnetism. Two unit quantities of 

 magnetism concentrated at points unit distance apart repel each other with unit 

 force. The dimensional formula is thus the same as for [force X length 2 X in- 

 ductive capacity] or M^UT" 1 ? 1 , and the conversion factor is 



2. Density of Surface Distribution of Magnetism. This is measured 

 by quantity of magnetism per unit area, and the dimension formula is therefore 

 the ratio of the expressions for magnetic quantity and for area, or M i Lr i T~ 1 P } , 

 which gives the conversion factor 



3. Magnetic Force at a Point, or Intensity of Magnetic Field. The 

 number for this is the ratio of the numbers representing the magnitudes of the 

 force on a magnetic pole placed at the point and the magnitude of the magnetic 

 pole. 



The dimensional formula is therefore the ratio of the expressions for force and 

 magnetic quantity, or 



M iJJ T -lpi 



and the conversion factor 



4. Magnetic Potential. The magnetic potential at a point is measured by 

 the work which is required to bring unit quantity of positive magnetism from zero 

 potential to the point. The dimensional formula is thus the ratio of the formula 

 for work and magnetic quantity, or 



which gives the conversion factor 



5. Magnetic Moment. This is the product of the numbers for pole 

 strength and length of a magnet. The dimensional formula is therefore the pro- 

 duct of the formulae for magnetic quantity and length, or M^T" 1 ?*, and the con- 

 version factor 



6. Intensity of Magnetization. The intensity of magnetization of any por- 

 tion of a magnetized body is the ratio of the numbers representing the magni- 



