The cube is constructed as a hollow body with plates which are held in position by an internal or external welded or glued frame. The plates are made of a material of low thermal expansion coefficient. A temperature sensor is fitted inside the cube. The balls are connected to rods along the cube diagonals, so that all balls can be contacted by gauges along the three orthogonal axes of the cube. The positions of the ball centres are calibrated by a self-centring comparator which is zeroed by a calibration ball rod.
