Directional Gyro is another vacuum driven gyroscope. It looks much like a compass. A major difference it has with the compass is that it doesn’t rely on the earth’s magnetic field to operate.
When the gyroscope is spinning, it has a principle of remaining rigid in space. That is the spinning wheel will resist any change in position. The DG takes advantage of that principle.
When an airplane is turning, the gyroscope will resist moving with the turn. The energy used to resist the turn instead moves the compass card which will indicate the heading of the airplane.
DG’s are used because they are not effected by magnetic disturbances or have turning errors inherent to the compass.