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ELECTROSTATIC SYSTEM 



Capacitance of an insulated conductor is proportional to the ratio of the 

 quantity of electricity in a charge to the potential of the charge. The dimen- 

 sional formula is the ratio of the two formulae for electric quantity and 

 potentiator [M^T^^M^T^K-*] or [LK]. 



Conductance of any part of an electric circuit, not containing a source of 

 electromotive force, is the ratio of the current flowing through it to the differ- 

 ence of potential between its ends. The dimensional formula is the ratio of the 

 formulae for current and potential or {APUT^K^/M^Ln-'K-'} or [LT'K]. 



Electrical conductivity, like the corresponding term for heat, is quantity 

 per unit area per unit potential gradient per unit of time. The dimensional 

 formula is [M^UT^ /L 2 (M } L T^"* /L)T] or [T'K]. 



Electric current (statampere-unit quantity) is quantity of electricity flow- 

 ing through a cross section per unit of time. The dimensional formula is the 

 ratio of the formulae for electric quantity and for time or [M h L*T~ 1 K i /T] or 



Electric field intensity strength at a point is the ratio of the force on a 

 quantity of electricity at a point to the quantity of electricity. The dimensional 

 formula is therefore the ratio of the formulae for force and electric quantity or 



[mlt-^/m^ot 1 ^] or [m^^t^k-*]. 



Electric potential difference and electromotive force (emf ) (statvolt- 

 work = 1 erg). — Change of potential is proportional to the work done per unit 

 of electricity in producing the change. The dimensional formula is the ratio of 

 the formulae for work and electrical quantity or [ML 2 T 2 /M i OT~ 1 K i ] or 

 [M i L l T- L K~ 1 ]. 



Electric surface density of an electrical distribution at any point on a sur- 

 face is the quantity of electricity per unit area. The dimensional formula is the 

 ratio of the formulae for quantity of electricity and for area or [APL'* f 1 ^]. 



Quantity of electricity has the dimensional formula [M i L*T~ l K i ], as 

 shown above. 



Resistance is the reciprocal of conductance. The dimensional formula is 

 [L^TK- 1 ]. 



Resistivity is the reciprocal of conductivity. The dimensional formula is 

 [TK- 1 ]. 



Specific inductive capacity is the ratio of the inductive capacity of the 

 substance to that of a standard substance and therefore is a number. 



Exs.— Find the factor for converting quantity of electricity expressed in ft-grain-sec 

 units to the same expressed in cgs units. The formula is Vm^Ht' 1 ^], in which m = 0.0648, 

 / = 30.48, t = 1, k — 1 ; the factor is 0.0648* X 30.48?, or 42.8. 



Find the factor required to convert electric potential from mm-mg-sec units to cgs 

 units. The formula is [m*/ } r7,r } ], in which m= 0.001, / = 0.1, t = \, k — 1; the factor is 

 0.001* X0.1 } , or 0.01. 



Find the factor required to convert electrostatic capacity from ft-grain-sec and specific- 

 inductive capacity 6 units to cgs units. The formula is [Ik] in which / = 30.48, k = 6; 

 the factor is 30.48 X 6, or 182.88. 



SMITHSONIAN PHYSICAL TABLES 



