XXX11 INTRODUCTION. 



20. Electro-kinetic Momentum. The number for this is the product of 

 the numbers for current and for electro-kinetic inertia. The dimensional formula 

 is therefore the product of the formulae for these quantities, or M^T" 1 ?"* X LP 

 = M*UT- 1 P*, and the conversion factor is 



21. Electromotive Force at a Point. The number for this quantity is 

 the ratio of the numbers for electric potential or electromotive force as given in 

 12, and for length. The dimensional formula is therefore M i L i T~ 2 P J , and the 

 conversion factor 



22. Vector Potential. This is time integral of electromotive force at a 

 point, or the electro-kinetic momentum at a point. The dimensional formula 

 may therefore be derived from 21 by multiplying by T, or from 20 by dividing 

 by L. It is therefore M*!,*!"" 1 ?*, and the conversion factor 



23. Thermoelectric Height. This is measured by the ratio of the num- 

 bers for electromotive force and for temperature. The dimensional formula is 

 therefore the ratio of the formulae for these two quantities, or M i L i T~ 2 P i ~ 1 , and 

 the conversion factor 



24. Specific Heat of Electricity. This quantity is measured in the same 

 way as 23, and hence has the same formulas. 



25. Coefficient of Peltier Effect. This is measured by the ratio of the 

 numbers for quantity of heat and for quantity of electricity. The dimensional 

 formula is therefore 



and the conversion factor 



EXAMPLES OF CONVERSION IN ELECTROMAGNETIC UNITS. 



(a) Find the factor required to convert intensity of magnetic field from foot 

 grain minute units to c. g. s. units. 



By (3) the formula is w*/"*/" 1 /"*, and in this case m = 0.0648, /= 30.48, / = 

 60, and/ = i ; .*. the factors = 0.0648* X 30.48"* X 6o~ 1 = 0.00076847. 



Similarly to convert from foot grain second units to c. g. s. units the factor is 

 0.0648* X 30.48"* = 0.046 1 08. 



() How many c. g. s. units of magnetic moment make one foot grain second 

 unit of the same quantity ? 



By (5) the formula is #z*/*/~~^*, and the values for this problem are m = 0.0648, 

 /= 30.48, t= i, and/ = i ; .'. the number = 0.0648* X 30.48*= 1305.6. 



(c) If the intensity of magnetization of a steel bar be 700 in c. g. s. units, what 

 will it be in millimetre milligramme second units ? 



