INTRODUCTION. XXVII 



2. Electric Surface Density and Electric Displacement. — The density 

 of an electric distribution at any point on a surface is measured by the quantity 

 per unit of area, and the electric displacement at any point in a dielectric is mea- 

 sured by the quantity displaced per unit of area. These quantities have therefore 

 the same dimensional formuhi, namely, the ratio of the formulae for quantity of 

 electricity and for area or MiL-^T-^K*, and the conversion factor mH-H^kK 



3. Electric Force at a Point, or Intensity of Electric Field. — This is 

 measured by the ratio of the magnitude of the force on a quantity of electricity at 

 a point to the magnitude of the quantity of electricity. The dimensional formula 

 is therefore the ratio of the formulce for force and electric quantity, or 



MLT^ ^ M^L-*T-iK-^, 



M»L3T-^K* 

 which gives the conversion factor jti'l~^t~'^k~^. 



4. Electric Potential and Electromotive Force. — Change of potential 

 is proportional to the work done per unit of electricity in producing the change. 

 The dimensional formula is therefore the ratio of the formulae for work and elec- 

 tric quantity, or 



^^^'^"' = M^L*T-^K-^ 

 M^L^T-^K* 



which gives the conversion factor ;«*/V~*>^~*. 



5. Capacity of a Conductor. — The capacity of an insulated conductor is 

 proportional to the ratio of the numbers representing the quantity of electricity in 

 a charge and the potential of the charge. The dimensional formula is thus the 

 ratio of the two formulce for electric quantity and potential, or 



MJL^T-^K^ _ ^^ 



which gives Ik for conversion factor. When K is taken as unity, as in the ordinary 

 units, the capacity of an insulated conductor is simply a length. 



6. Specific Inductive Capacity. — This is the ratio of the inductive cap?c- 

 ity of the substance to that of a standard substance, and hence the dimensional 

 formula is K/K or i.* 



7. Electric Current. — Current is quantity flowing past a point per unit of 

 time. The dimensional formula is thus the ratio of the formulae for electric quan- 

 tity and for time, or 



M!L^' = M.L'T-'K., 



and the conversion factor ;«-/'/~^^*. 



* According to the ordinary definition referred to air as standard medium, the specific inductive 

 capacity of a substance is K, or is identical in dimensions with what is here taken as inductive ca- 

 pacity. Hence in that case the conversion factor must be taken as i on the electrostatic and as 

 h'^fi on the electromagnetic system. 



