INTRODUCTION. XXV 



mula ml~ l t~ l 6j where m . 064 799, /= 30.48, and /= i, and is therefore = 

 .064799/30.48 = 2.126 X io~ 8 . 



(c) Find the relation between the units stated in (ft) for emissivity. 

 In this case the conversion formula is w/" 2 /"" 1 , where ml and / have the 

 same value as before. Hence the number of the latter units in the former is 

 2 = 6.975 X io~ 6 . 



(d) Find the number of centimetre gramme second units in the inch grain 

 hour unit of emissivity. 



Here the formula is ml~*t~ l , where m 0.064 799 ^=2.54, and ^ = 3600. 

 Therefore the required number is 0.064 799/ 2 -54 2 X 3600 = 2.790 X io~*. 



(e) If Joule's equivalent be 776 foot pounds per pound of water per degree 

 Fahrenheit, what will be its value in gravitation units when the metre, the 

 kilogramme, aud the degree Centigrade are units ? 



The conversion factor in this case is ,,_a or I0~ l , where / = .3048 and 

 ff- l = i.S-, .'. 776 X .3048 X 1.8 = 425.7. 



(/) If Joule's equivalent be 24832 foot poundals when the degree Fahren- 

 heit is unit of temperature, what will be its value when kilogramme metre 

 second and degree-Centigrade units are used ? 



The conversion factor is Pr*0~ l , where /= .3048, t = i, and 0~ l = 1.8 ; 



.-. 24832 x r-r 2 d~ l = 24832 x .3048' x 1.8 = 4152.5. 



In gravitation units this would give 4152.5/9.81 = 423.3. 



ELECTRIC AND MAGNETIC UNITS. 



There are two systems of these units, the electrostatic and the electromagnetic 

 systems, which differ from each other because of the different fundamental suppo- 

 sitions on which they are based. In the electrostatic system the repulsive force 

 between two quantities of static electricity is made the basis. This connects force, 



quantity of electricity, and length by the equation /=a 22l, where / is force, a a 



quantity depending on the units employed and on the nature of the medium, q and 

 q l quantities of electricity, and / the distance between q and q t . The magnitude of 

 the force / for any particular values of q, q t and / depends on a property of the 

 medium across which the force takes place called its inductive capacity. The in- 

 ductive capacity of air has generally been assumed as unity, and the inductive 

 capacity of other media expressed as a number representing the ratio of the induc- 

 tive capacity of the medium to that of air. These numbers are known as the spe- 

 cific inductive capacities of the media. According to the ordinary assumption, 

 then, of air as the standard medium, we obtain unit quantity of electricity when 

 in the above equation y = ? { , and/, a, and / are each unity. A formal definition 

 is given below. 



In the electromagnetic system the repulsion between two magnetic poles or 



