The apparatus of a Michelson-Morley experiment measures the instantaneous difference in the speed of light in two perpendicular directions and repeats this measurement over a variety of orientations. These results are combined to show the velocity of a presumed ether through the solar system.
The general approach is to direct light along two perpendicular arms and interferometrically measure the difference in speeds between the light-paths in the arms. The arms are horizontal and rotate about a vertical axis a few times a minute. The horizontal plane of rotation sweeps through a large portion of the sky each day. These rotations provide the variety of orientations needed for calculations.
The sense element of the arms is an optical fiber which lies in a tray, which in turn lies in a clear acrylic tube 3 inches diameter. The straight length, end to end, of the fiber is about 16 feet, a bit longer than the straight light path of Dayton Miller's apparatus. One arm includes two 8 foot lengths of tube joined to the hub. The sixteen foot arm's tubes are supported by beams that are aluminum extrusions. The rotation of the arms is driven by a servo-motor attached to the ceiling of the lab. The motor-shaft is connected to the hub.
The hub holds a number of components in addition to the two 16 foot arms. This includes a DFB diode laser, two photodetectors, a data logging computer, and fiber optic components. The components that produce heat are placed above fiber components to avoid potential thermal variations in the fiber that might affect the interferometric signal.