Credit: Bob Kahn/Stanford University/Gravity Probe B

Figure 2: A gyroscope aboard the Gravity Probe B satellite. Each gyroscope is a fused silica rotor about the size of a ping-pong ball, machined to be spherical and homogeneous to tolerances better than a part per million, and coated with a thin film of superconducting niobium. The gyroscope assembly sat in a dewar of $2440$ liters of superfluid helium and was held at $1.8$ degrees Kelvin.