in the apparatus for applying a polymer film to a support as the film and support pass between a heating roller and a pressure roller the heating roller is provided with a freely movable electrical contact comprised of a disk extending into a pool of mercury at one end of the heating roller to accommodate longitudinal heat expansion of the roller to prevent radial variations in the heating element with consequent variations in heating patterns . the supply reel for the polymer film is provided with a spring bias friction brake having a readily disconnectable coupling with the supply reel to facilitate reel changing .