The investigation of the currents induced in a conductor by the passage of a mass of magnetic material over it is a matter of some considerable interest. In the simple case where the mass is in the form of a spherical shell, the problem can be investigated mathematically, and the motion of the spot of a galvanometer connected to such a conductor can be deduced. It has occurred to the writer to compare the results of a mathematical investigation with those of experiments carried out in the laboratory on a small scale. It is the object of the present paper to give a summary of both the theory and the experimental confirmation. Consider an iron shell (fig. 1) of internal and external diameters
a
and
b
, and of permeability
μ
, passing with velocity
v
over an infinitely long straight conductor at depth
d
below the level of its centre. Take axes through the centre, the Y-axis being vertical and the X- and Z-axes being horizontal, the one perpendicular, the other parallel to the direction of the conductor. Let
l, m, n
be the direction cosines of F, the earth’s field, and let
α
be the angle between the Z-axis and the direction of motion of the shell.