^Compass & dip circle
A compass needle consists of a magnetic needle is free to swing (oscillate or deflect) in a horizontal plane about a vertical axis.
A compass needle when placed in a horizontal plane & free to move aligns itself parallel to BH.
At the poles, the magnetic field lines are vertically so that the horizontal component is negligible & the compass needle can point along any direction & thus can’t be used as a direction finder. In that case we use a dip circle.