Geodesic measurements using an invar wire

The album shows two members of the Survey Group of CERN's AR Division, D. Bois (left) and A. Gressot, checking the position of one of the proton-synchrotron magnets. They are using an instrument invented by the Group and perfected over the last few years to the extent that the 100 magnets of the accelerator can now be surveyed, with a precision of better than a tenth of a millimetre, by three men in only two days. The conventional method previously used occupied four men for eight days. Named 'Distinvar', the new apparatus, like the old, employs calibrated wires made of invar, an alloy of iron and nickel with a negligible coefficient of thermal expansion. The wires are of various lengths, corresponding to the nominal distances to be measured; one end is fixed in a 'balance', which serves to define the position of the end and to apply a fixed tension to the wire, the other is fixed to a micrometer screw which is used to measure the precise difference between the length of the wire and the distance being measured. The two end pieces are accurately mounted in special sockets, in the case of the PS one in the concrete survey pillar and the other on the magnet. Calibration of the wires is carried out on a comparator - constructed by the Société Genevoise d'Instruments de Physique and installed in one of the PS radial tunnels - which is regarded as the most up-to-date in the world. The Distinvar, which has many possible applications outside the field of accelerator technology, has been described by the Leader of the Survey Group, J. Gervaise, in CERN reports 64-16 (French) and 64-39 (English).