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SI BASE UNITS

 
         
    Quantity Electric current  
 
    Unit

ampere

  
 
 
    Symbol A  
 
    Definition

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.

    
 
    Comments
  The above definition of the ampere was introduced in 1948, abolishing the pre-exisiting "international" definition of current.  
   
The effect of the definition is to set the permeability of vacuum to 4 π × 10-7 H m-1 .  
   
The ampere is most accurately measured through measuring combinations of the watt, the volt and the ohm. The watt and the volt can be obtained from balances in which electrical forces are compared with mechanical forces. The ohm is best measured using a Thompson-Lampard capacitor whose value can be changed by the linear displacement of a guard electrode. From any two of these three measurements the ampere can be obtained to an accuracy of a few parts in 107.  
 
    References (A) The International System of Units , Bureau International des Poids et       Mesures, 7 th edition, 1998  
         


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