XXXii 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 formulse for these quantities, or M-L-T-^P-* X LP 

 ■=^ M^L-T^^P*, and the conversion factor is ;«V-/~^/-. 



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 ;«-/V~"^-. 



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 wV-/-^". 



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^0~S and 

 the conversion factor m^-l^-i~'J}^6~^. 



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 



,^f , = M'L-^P^0, 

 M^L^P-i ' 



and the conversion factor mrl~^p^B. 



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 m^t-H-'^p~\ and in this case in = 0.0648, /= 30.48, / = 

 60, and/ = I ; .'. the factors = 0.0648* X 30.48"^ X 60"^= 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. 



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

 unit of the same quantity ? 



By (5) the formula is wV*/-y, and the values for this problem are m = 0.0648, 

 / = 30.48, / = I, and/ = I ; .-. the number = 0.0648* X 30-48- = i3o5-6- 



(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 ? 



I 



