OBERKOCHEN, Germany -- Carl Zeiss has built what it calls the world's largest ring-laser gyro. Designed for measuring the Earth's rotation, the device will be installed in a subterranean cave on the Banks Peninsula in New Zealand early this year. The project was commissioned by the Institute of Applied Geodesy in Frankfurt in cooperation with the Technical University of Munich and is valued at roughly DM 750,000.The gyro's rotational resolution of 1:10,000,000 over long time spans allows highly accurate detection of fluctuations in the Earth's rotation -- an attribute of particular interest to geophysicists seeking a better understanding of the Earth's interior structure.Now geophysicists will be able to draw conclusions on displacements inside the Earth, the continental drift and the incidence of earthquakes. The laser approach exploits the fact that typical seismographs measure only linear seismic shocks, whereas a ring-laser gyro indicates the different rotational components of the seismic shocks.The ring-laser gyro's body consists of a 1.2 3 1.2-m block of Zerodur 180 mm thick and weighing approximately 600 kg. The block was produced by Schott Glaswerke in Mainz and brought to Zeiss in Oberkochen for machining. The block has four longitudinal bores for the laser beam, each 1 m long. Deflecting mirrors at each corner produce a closed, square resonator in which a laser beam can travel in both directions. Zerodur was chosen for the block material for its thermal stability.Zeiss designed a polishing process that brought the oblique surfaces to l/4, permitting the mirrors to be mounted without adhesives.