416 The Magnetic Field produced by Electric Currents [CH. xin 



483. Law of Signs. If an observer is imagined to stand on that side of 

 the "equivalent magnetic shell" which contains the negative poles, the 

 current flows round him in the same direction as that in which the sun 

 moves round an observer standing on the earth's surface in the northern 

 hemisphere. 



We can also state the law by saying that to drive an ordinary right- 

 handed screw (e.g. a cork-screw) in the direction 

 of magnetisation of the shell, the screw would 



c 



have to be turned in the direction of the 

 current. 



Current 

 The law of signs expresses a fact of nature, not a -f- -f- , -f + 



mathematical convention. At the same time, it must be 

 noticed that the law does not express that nature shews 



any preference in this respect for right-handed over left- 



_ , "" , . . , Direction of Magnetisation 



handed screws. Two conventions have already been made . equivalent shell 



in deciding which are to be called the positive directions FIG 12Q , 



of current and of magnetisation, and if either of these 



conventions had been different, the word " right-handed " in the law of signs would have 

 had to be replaced by " left-handed." 



Electromagnetic Unit of Current. 



484. If i is the strength of the current flowing in a circuit, and <j) the 

 strength of the equivalent magnetic shell, then 



</> = ki, 



where & is a constant, which is positive if the law of signs just stated has 

 been obeyed in determining the signs of < and i. 



In the system of units known as Electromagnetic, we take k=\, and 

 define a unit current as one such that the equivalent magnetic shell is of 

 unit strength. The strength of a current, in these units, is therefore 

 measured by its magnetic effects. Obviously the strength measured in this 

 way will be entirely different from the strength measured by the number of 

 electrostatic units of electricity which pass a given point. This latter method 

 of measurement is the electrostatic method. A full discussion of systems of 

 units will be given later ( 582); at present it may be stated that a current 

 which is of unit strength when measured electromagnetically in C.G.S. units is 

 of strength 3 x 10 10 (very approximately) when measured electrostatically. The 

 practical unit of current, the ampere, is, as already stated, equal to 3 x 10 9 

 electrostatic units of current, so that the electromagnetic unit of current is 

 equal to 10 amperes. 



