Joule kom til dette resultat eksperimentelt i [[1841]], ved at anvende et [[kalorimeter]] til at måle varme – og et [[galvanometer]] til at måle strømmen, med flere forskellige resistive elektriske kredsløb.
<ref>
{{cite book
 | title = A Treatise on Electricity and Magnetism
 | volume = II
 | edition = 2nd
 | author = James Clerk Maxwell
 | publisher = Oxford: Clarendon
 | year = 1881
 | page = 377
 | url = http://books.google.com/books?id=jAgXAAAAYAAJ&pg=PA377&dq=current+square+%22joule%27s+law%22+date:0-1900&lr=&as_brr=3&as_pt=ALLTYPES&ei=2mqpSbqkJYnikATjg_mPBA
 }}
</ref><ref>
{{cite book
 | title = A Text-book of Physics
 | author = William Watson and Herbert Moss
 | publisher = Longmans, Green, and Co
 | year = 1920
 | page = 708
 | url = http://books.google.com/books?id=EzVJAAAAIAAJ&pg=PA708&dq=%22joule%27s+law%22+%22proportional+to+the+square+of+the+current%22&lr=&as_brr=3&as_pt=ALLTYPES&ei=Mp2pSYv6IZvukQTzvo2CBA
 }}</ref>
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