Hall Effect enables the charge carrier concentration and mobility to be determined by experiment. Direction of the Hall Voltage in silver indicates negative charge carriers, which is in agreement with concepts of the model of the ‘free electron gas‘. Limitations of this model are shown by the so called ‘abnormal Hall Effect’ of tungsten. The experiment carried out under identical conditions for tungsten show the Hall Voltage to have about same magnitude but opposite direction as in silver. This can be explained by the ‘Energy Band diagram’. The tungsten atom has …….5s² 5p⁶ 5d⁴ 6s² electronic structure. When the atoms come close together to form the solid, the close lying states 5d and 6s broaden into bands, with s band broadening considerably more than the d band. This is because of the larger size of the s orbital. The figure schema- tically shows the allowed energies as a function of the interatomic distance. The number of allowed states is ten per atom in the d band and two in the s band. In tungsten there are six electrons to be shared between these two bands. The result is that at the interatomic distance in tungsten there are holes in the d band and electrons in the s band, making tungsten predominantly a hole conductor. This sort of mixed (electrons and holes) conduction is a general characteristic of transition metals. The apparatus consists of the following:

Description of the experimental set-up

The set-up consists of the following.

a) Hall Probe-Silver (HP-Ag)

b) Hall Probe-Tungsten (HP-W)

c) High Current Power Supply, Model PS-20A

d) Digital Microvoltmeter, DMV-001

Pole Pieces

75mm tappered to 25mm

Mag. Field

20KG at 6mm airgap

Energising Coils

Two of approx. 13W each

Power

0-90Vdc, 3A, for coils in series
0-45Vdc, 6A, for coils in parallel