指南针 The compass is introduced

A compass is a simple instrument discrimination range， also known as thecompass， according to \"record\" on the earliest records appeared in the Warring States period magnetic mountain area. The compass was formerly one of thefour great inventions of ancient China\"s compass， is the result of ancient Chinese labor people understanding of magnetic objects in the long-term practice.

The main components are mounted on a shaft can freely rotate the needle， the needle can remain in the tangent magnetic radial direction on the ground under the action of magnetic field， magnetic needle points to the geographical North pole pole， direction can be identified by this performance. Commonly used in navigation， geodesy， travel and military etc.