<P> The Hall effect is often used to measure the magnitude of a magnetic field . It is used as well to find the sign of the dominant charge carriers in materials such as semiconductors (negative electrons or positive holes). </P> <P> An important use of H is in magnetic circuits where B = μH inside a linear material . Here, μ is the magnetic permeability of the material . This result is similar in form to Ohm's law J = σE, where J is the current density, σ is the conductance and E is the electric field . Extending this analogy, the counterpart to the macroscopic Ohm's law (I = V ⁄ R) is: </P> <Dl> <Dd> Φ = F R m, (\ displaystyle \ Phi = (\ frac (F) (R)) _ (\ mathrm (m)),) </Dd> </Dl> <Dd> Φ = F R m, (\ displaystyle \ Phi = (\ frac (F) (R)) _ (\ mathrm (m)),) </Dd>

Who discovered that a changing magnetic field can produce an electric current