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H

There are two curious points about half-life that are worth remembering. First, it has the largest range of values of any known physical quantity. Secondly, there is no known way of changing the half-life of a specimen of a particular nuclide; high tem­ peratures, strong magnetic fields, etc. have no effect since the extranuclear electrons form an efficient shield. (See also decay constant, randomness and radioactive decay, exponential law o f radioactive decay.)

Hall e f f e c t : a difference in potential Vu set up in a conductor at right angles to the current flow direction by a magnetic field B. The diagram belows shows a thin rect­ angular conductor with the current / flowing parallel to the long side. A magnetic field of strength B is switched on at right angles to the face of the specimen with the largest area. The force on the current in the magnetic field causes it to be deflected downward; check this with Flemings left hand rule. The concentration of charge on the lower edge sets up a voltage that opposes further deflection of the moving charge. Equilibrium is reached when:

_ 67 - nqt

where n is the number of charge carriers per unit volume of conductor, q is the charge per carrier and t is the thickness of the specimen in the direction of the magnetic field.