XXX 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* T-^K or electric quantity ^ 



,M'L*T~^K~tp ' area X potential gradient X time 



^' L 



The conversion factor is t~'^k. 



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

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

 tively TK-^ 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 fiowing through it and the difference of potential between its ends. 

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

 tial, or 



MiL^T-^K^ _ ^^ 

 M^LiT-^K-^-^ ' 



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



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

 dimensional formula and the couversiou factor are respectively ly^'TK"' auJ 

 l-Hk-\ 



EXAMPLES OF CONVERSION IN ELECTROSTATIC UNITS, 



{a) find 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 ?«7'V^U'-, in which in this case Jti = 0.064S, /= 30.48, / = 

 I, and k^ 1 ; .-. the factor is 0.0648^ X 30.48^ = 4.2836. 



(d) Find the factor required to convert electric potential from millimeter milli- 

 gram second units to c. g. s. units. 



By (4) the formula is ;«-/-7~^/^~*, and in this case ?n = o.ooi, /= o.i, /*= i, and 

 /('=i; .•. the factor ^ O.OOI- X 0.1-:= 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 (s) the formula is //&, and in this case /= 30.48 and ^=^6; .'. the factor 

 = 30.48X6=182.88. 



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

 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. 



