Abstract: An automatic indexing mechanism for stepping one or more thermocouples through selected positions in a gas turbine engine comprises a quadrant plate having an arcuate row of apertures therein. A first arm is pivotally mounted on the plate and carries the one or more thermocouples. A further arm is pivoted mounted on the plate and also with respect to the first arm and has attached thereto a piston rod of a pneumatic piston and cylinder device whose body is fixed to the first arm. Stops actuated by piston and cylinder devices are mounted on the arms and are arranged to be engaged in the arcuate row of apertures in the plate. A pneumatic circuit and an electrical logic circuit control operation of the piston and cylinder devices to effect stepwise movement of the first arm relative to the quadrant plate.