XXVlll INTRODUCTION. 



8. Conductivity, or Specific * Conductance. — This, like the corresponding 

 term for heat, is quantity per unit area per unit potential gradient per unit of time. 

 The dimensional formula is therefore 



M^L' T-^K> _ ^1 j^ electric quantity ^ 



^M^UT~^K~^rY> ' area X potential gradient X time 



The conversion factor is /~*i. 



9. Specific * Resistance. — This is the reciprocal of conductivity as above 

 defined, and hence the dimensional formula and conversion factor are respec- 

 tively TK-i and fk-\ 



10. Conductance. — The conductance of any part of an electric circuit, not 

 containing a source of electromotive force, is the ratio of the numbers represent- 

 ing the current flowing through it and the difference of potential between its ends. 

 The dimensional formula is thus the ratio of the formulae for current and poten- 

 tial, or 



M^L^T-^K^ _ J ^_,^ 

 MiL^T-^K-i ' 



from which we get the conversion factor U~^k. 



11. Resistance. — This is the reciprocal of conductance, and therefore the 

 dimensional formula and the conversion factor are respectively L<"~'TK~^ and 



EXAMPLES OF CONVERSION IN ELECTROSTATIC UNITS, 



(a) Pind the factor for converting quantity of electricity expressed in foot grain 

 second units to the same expressed in c. g. s. units. 



By (i) the formula is m-Ih~^^-, in which in this case m = 0.0648, /= 30.48, / = 

 I, and ^ = 1 ; ,-. the factor is 0.0648^ X 30-48' = 4.2836. 



(^) Find the factor required to convert electric potential from millimetre milli- 

 gramme second units to c. g. s. units. 



By (4) the formula is M^-/-f~'^k~\ and in this case m = 0.00 1, /= o.i, /= i, and 

 k=i; .'. the factor = 0.001* X o- 1^ = 0.01. 



(c) Find the factor required to convert from foot grain second and specific in- 

 ductive capacity 6 units to c. g. s. units. 



By (5) the formula is /k, and in this case /= 30.48 and k = 6 ; .'. the factor 

 = 30.48 X 6 = 182.88. 



* The term "specific," as used here and in 9, refers conductance and resistance to that between 

 the ends of a bar of unit section and unit length, and hence is different from the same term in 

 specific heat, specific inductivity, capacity, etc., which refer to a standard substance. 



