THE MEASUREMENT OF CURRENT 



93 



small metal flag attached to the movable system which can be 

 manipulated by means of a forked lever operated from outside 

 the case. 



Suppose that the movable weight is set at zero and that the 

 balance is in equilibrium when no current is passing. On the 

 passage of the current the equilibrium will be upset and the right- 



. !'.. Kelvin current balance. 



hand end of the beam will rise. To restore the equilibrium it is 

 necessary to increase the turning moment due to the weights. 

 This is accomplished by displacing the movable weight toward 

 the right. The change in the total moment due to the weights is 

 proportional to the displacement of the movable weight from its 

 zero position, and this must be equal to the moment due to the 



