The Lorenz number, which was assumed to be a universal factor, was lat
er found to be deviating from its theoretical Sommerfeld value. Our un
derstanding of the physics of the Weidemann-Franz Lorenz law is based
on a large number of investigations. In view of its importance in the
understanding of thermal and electrical conductivity (particularly the
electronic contribution), an attempt has been made to review the expe
rimental determination of Lorenz number in metals, semi-metals, alloys
and degenerate semiconductors.