INTRODUCTION xli 



Resistivity is the reciprocal of conductivity as just defined. The dimensional 

 formula is \_DT~ l ix~]. 



Self-inductance is for any circuit the electromotive force produced in it by 

 unit rate of variation of the current through it. The dimensional formula is 

 the product of the formulae for electromotive force and time divided by that 

 for current or IM^LIT~ 2 ^ x T + M*L*7^V _i ] or [L^]. 



Mutual Inductance of two circuits is the electromotive force produced in one 

 per unit rate of variation of the current in the other. The dimensional formula 

 is the same as for self-inductance. 



Electric Field Intensity is the ratio of electric potential or electromotive force 

 and length. The dimensional formula is [il/*Z,»jT" 2 jU*]. 



Magnetic Reluctance is the ratio of magnetic potential difference to magnetic 

 flux. The dimensional formula is [L _1 /i -1 ]. 



Thermoelectric Power is measured by the ratio of electromotive force and 

 temperature. The dimensional formula is [\l/'*Z.- ; 7 , ~ 2 /^9 _1 3. 



Coefficient of Peltier Effect is measured by the ratio of the quantity of heat 

 and quantity of electricity. The dimensional formula is [ML 2 T~ 2 /M*Lljx~'2 or 

 \_M$UT~ 2 ijy\, the same as for electromotive force. 



Exs. — Find the factor required to convert intensity of magnetic field from ft.-grain-min. 

 units to c.g.s. units. The formula is [_mil~U~ l fi~^2\ m - 0.0648, / = 30.48, / = 60, and fi=i; 

 the factor is 0.06483 X 3o.4S~a, or 0.046108. 



How many c.g.s. units of magnetic moment make one ft.-grain-sec. unit of the same quan- 

 tity? The formula is [wl/i/ -1 /**]; m = 0.0648, /= 30.48, / = 1, and /x = 1; the number is 

 0.0648a X 30.48?, or 1305.6. 



If the intensity of magnetization of a steel bar is 700 in c.g.s. units, what will it be in mm- 

 mg-sec. units? The formula is [miHr' 1 /JLi2> m - 1000, I = 10, t = 1, /x = 1; the intensity is 

 700 X ioooj" x ioa, or 70000. 



Find the factor required to convert current from c.g.s. units to earth-quadrant-io -11 gram- 

 sec, units. The formula is [ffii/ar'ju's]; m = io 11 , / = io -9 , yu = 1; the factor is io 1 * 1 x io - ?, 

 or 10. 



Find the factor required to convert resistance expressed in c.g.s. units into the same expressed 

 in earth-quadrant- io -11 gram-sec. units. The formula is [lt~ l fJL}\ / = io -9 , t = 1, (jl = 1; the 

 factor is io -9 . 



FUNDAMENTAL STANDARDS 



The choice of the nature of the fundamental quantities already made does 

 not sufficiently define the system for measurements. Some definite unit or 

 arbitrarily chosen standard must next be taken for each of the fundamental 

 quantities. This fundamental standard should have the qualities of perma- 

 nence, reproducibility and availability and be suitable for accurate measures. 

 Once chosen and made it is called the primary standard and is generally kept 

 at some central bureau, — for instance, the International Bureau of Weights 

 and Measures at Sevres, France. A primary standard may also be chosen and 

 made for derived units (e.g., the international ohm standard), when it is simply 

 a standard closely representing the unit and accepted for practical purposes, 

 its value having been fixed by certain measuring processes. Secondary or refer- 



