xxxiv 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*L^T~^P~^ X LP 

 = M-L-T~^P^, and the conversion factor is mHH~'^p^, 



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-L*T~^P-, and the 

 conversion factor w-/-^~^/*. 



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^L-T~^P^, and the conversion factor ;?/-/V~^-. 



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*L^T~"P^©~^, and 

 the conversion factor mHH~'^f'B~^. 



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

 way as 23, and hence has the same formulae. 



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 



_^ = M^L-iP^©, 

 M^L^P-i ' 



and the conversion factor m^l~^p^9. 



EXAMPLES OF CONVERSION IN ELECTROMAGNETIC UNITS. 



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

 gi'ain minute units to c. g. s. units. 



By (3) the formula is in^t~H~^p~^-, 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"- = 0.046 108. 



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

 unit of the same quantity .'' 



By (5) the formula is »?V*/~^/-, and the values for this problem are pi = 0.0648, 

 ,/= 30.48, t = I, and/ = I ; .*. the number = 0.0648* X 30-48^= 1305.6. 



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

 will it be in millimeter milligram second units ? 



