INTRODUCTION. XXXIU 



By (6) the formula is m^fir^J>^, and in this case m =. looo, /= lo, /= i, and 

 / = I ; .•. the intensity = 700 X 1000* X 10* = 70000. 



(d) Find the factor required to convert current strength from c. g. s. units to 

 earth quadrant io~" gramme and second units. 



By (9) the formula is /«W~^/~*, and the values of these quantities are here m = 

 io^\ /= Io~^ /= I, and/ = i ; .-. the factor = lo^i X io~»= 10. 



(<f) Find the factor required to convert resistance expressed in c. g. s. units into 

 the same expressed in earth-quadrant io~" grammes and second units. 



By (14) the formula is /i~^/>, and for this case /== io~®, / = i, and / = i ; 

 .'. the factor = io~^. 



(/) Find the factor required to convert electromotive force from earth-quadrant 

 io~" gramme and second units to c. g. s. units. 



By (12) the formula is m-P^~^J>^, and for this case m = io~", /= 10®, /= i, 

 and/ = I ; .*. the factor = 10'. 



PRACTICAL UNITS. 



In practical electrical measurements the units adopted are either multiples or 

 submultiples of the units founded on the centimetre, the gramme, and the second 

 as fundamental units, and air is taken as the standard medium, for which K and P 

 are assumed unity. The following, quoted from the report to the Honorable the 

 Secretary of State, under date of November 6th, 1893, by the delegates repre- 

 senting the United States, gives the ordinary units with their names and values 

 as defined by the International Congress at Chicago in 1893 : — 



" Resolved, That the several governments represented by the delegates of this 

 International Congress of Electricians be, and they are hereby, recommended to 

 formally adopt as legal units of electrical measure the following : As a unit of re- 

 sistance, the international ohm, which is based upon the ohm equal to 10^ units of 

 resistance of the C. G. S. system of electro-magnetic units, and is represented 

 by the resistance offered to an unvarying electric current by a column of mercury 

 at the temperature of melting ice 14.4521 grammes in mass, of a constant cross- 

 sectional area and of the length of 106.3 centimetres. 



" As a unit of current, the international ampere, which is one tenth of the unit of 

 current of the C. G. S. system of electro-magnetic units, and which is represented 

 sufficiently well for practical use by the unvarying current which, when passed 

 through a solution of nitrate of silver in water, and in accordance with accom- 

 panying specifications,* deposits silver at the rate of 0.001118 of a gramme per 

 second. 



* " In the following specification the term ' silver voltameter' means the arrangement of appara- 

 tus by means of which an electric current is passed through a solution of nitrate of silver in water. 

 The silver voltameter measures the total electrical quantity which has passed during the time of 

 the experiment, and by noting this time the time average of the current, or, if the current has been 

 kept constant, the current itself can be deduced. 



" In employing the silver voltameter to measure currents of about one ampere, the following 

 arrangements should be adopted : — 



