The story of the Hall effect begins with a mistake made by James Clerk Maxwell. In the first edition of his Treatise on Electricity and Magnetism, which appeared in 1873, Maxwell discussed the deflection of a current by a magnetic field. He then said: “It must be carefully remembered that the mechanical force which urges a conductor … acts, not on the electric current, but on the conductor which carries it.” If readers are puzzled by this assertion, they should be.

In 1878, Edwin Hall, a student at Johns Hopkins University, was reading Maxwell for a class taught by Henry Rowland. Hall asked Rowland about Maxwell’s remark. The professor replied that he “doubted the truth of Maxwell’s statement and had sometime before made a hasty experiment … though without success.” 1  

Hall made a fresh start and designed a different experiment, aimed at measuring, instead, the magneto-resistance—that is, the...

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