Definition of thumb–and–finger rule
: either of two statements in electromagnetism: when the fingers of the right hand are bent so that the thumb points in the direction of the decrease in flux in a magnetic circuit, the fingers will point in the direction of the induced voltage in the electric circuit linked with the magnetic circuit; and when the fingers of the right hand are bent so that they follow the direction of the flow of current through a helical or coil conductor, the thumb will point in the direction of the flux through the helix or coil — compare left-hand rule right-hand rule
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