ELECTRICITY. 43 



Another way of regarding the practical units is to consider them 

 as derived from subsidiary units of length, mass, and time. The 

 unit of length (A) is taken as a quarter of the terrestrial 



meridian or about 10 9 cms. The unit of mass (u.) is taken as ^ n .- 



10 U 



gram or 10- 11 of the C.G.S. unit of mass. The unit of time r is 

 still taken as the second. 



Hence the corresponding practical unit of force would be Ajttr- 

 dynes or T ^ dyne, and the practical unit of work would be A 2 /xr- 2 

 ergs or 10 7 ergs. 



(57) PKACTICAL STANDARDS OF RESISTANCE. 



By Ohm's Law the current is equal to the electromotive force 

 maintaining it divided by all the resistance in the circuit 



Of these three quantities the easiest to measure is the resistance, 

 and hence on the standard of resistance all the other practical units 

 depend. 



The Electrical Congress at Paris in 1884 defined the legal ohm 

 to be the resistance of a column of mercury at C. 11 s. mm. in 

 section and 106 cm. long. It is rather less than 10 9 C.G.S. units 

 or an earth quadrant (10 9 cm.) per second. Siemens had previously 

 proposed the use of a similar column of mercury 100 cm. long. 



The B.A. unit of resistance is only equal to 0' 98655 x 10 

 C.G.S. units. 



The volt varies with the value of the ohm assumed, but llio 

 ampere remains 10- T C,G.S. 



