The q₂₁ pendulum consisting of four identical cylindrical test mass made of red copper, with essentially the same mass. They are positioned on a circular pendulum tray with a thickness of 2.5 mm and a radius of 40 mm. The numbers 1, 2, 3, and 4 mark the four-test mass, respectively. An aluminum rod with a length of 110 mm passes through the center of the tray, and two reflecting mirrors are installed on its bottom and top symmetrically. The main body of the pendulum is suspended by a torsion fiber with a length of 1000 mm and a diameter of 25 μm, which connects with the aluminum rod by a clamp. The torsional constant of the fiber k equals 6.2 × 10 Nm/rad. The total mass of the q₂₁ pendulum is 70 g, and the equals 35.7(3) g⋅cm .