The same effect was produced when, on holding the helix in the line of dip, a bar of iron was thrust into it.

Here, however, the earth acted on the coil through the intermediation of the bar of iron.

He abandoned the bar and simply set a copper plate spinning in a horizontal plane; he knew that the earth's lines of magnetic force then crossed the plate at an angle of about 70degrees.

When the plate spun round, the lines of force were intersected and induced currents generated, which produced their proper effect when carried from the plate to the galvanometer.

'When the plate was in the magnetic meridian, or in any other plane coinciding with the magnetic dip, then its rotation produced no effect upon the galvanometer.' At the suggestion of a mind fruitful in suggestions of a profound and philosophic character--I mean that of Sir John Herschel--Mr.Barlow, of Woolwich, had experimented with a rotating iron shell.