By placing one of the coils, A, in the block, D, then placing in the
cavity in the top of the block the compass, with the line marked N S
arranged at right angles to the axis of the coil, a serviceable
galvanometer will be formed (Fig. 8). By turning the galvanometer so
that the needle will point north and south without the current
passing, with N underneath one end of the needle, and then connecting
the poles of the battery with the terminals of this galvanometer, a
deflection of the compass needle will be produced, the direction of
which depends upon the direction of the current.