XXVIU 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^ _ -p-ijr- Qj. electric quantit y 



2M-L-T~^K~'^ ' area X potential gradient X time 



The conversion factor is f'^k. 



g. 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 tk-^. 



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 



from which we get the conversion factor It~'^k~^. 



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

 dimensional formula and the conversion factor are respectively L~^TK and /~VX'. 



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'-lk~^k^; in which in this case vi = 0.0648, /= 30.48, / = 

 I, and /I' = I ; .-. the factor is 0.0648* X 3°-4^- = 4-2836. 



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

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



By (4) the formula is w^/'/'U'"^, and in this case ;;/ = o.ooi, /^ o.i, /= i, and 

 /l'=i; .-. the factor = O.OOI- X o-i- = o.oi. 



(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 //t, and in this case /=: 30.48 and /: = 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. 



