How can the rigidity of a gyro be improved?

Increasing the RPM (revolutions per minute) of a gyro enhances its rigidity due to a phenomenon called gyroscopic stability. As the rotational speed of the gyro increases, the angular momentum also increases. This increased angular momentum provides greater resistance to changes in the orientation of the gyro, thereby improving its rigidity.

In practical terms, a gyro that spins at a higher speed will maintain its position more effectively against external forces or torques, making it more stable and reliable for navigation and guidance applications. Thus, increasing the RPM is a direct method to enhance the rigidity and stability of the gyro, allowing for more precise and accurate measurements.

The other options do not contribute positively to the rigidity of a gyro in the same way. Decreasing the rotor size or using a lighter material could diminish the angular momentum and thereby reduce stability, while expanding the rotor diameter does not inherently lead to improved rigidity without increased RPM.