A testing chamber for electrically testing a sample of insulating material or materials in sheet form, under controllable conditions of pressure and temperature. The pressure resistant chamber is air and liquid tight to permit testing of oil impregnated samples unexposed to harmful air. A frame is positioned within the chamber. A supply reel storing a test sample of insulating material or materials and a takeup reel for receiving the test sample drawn from the supply reel are both rotatably mounted on the frame. The supply and takeup reels are separated from each other to form a testing station therebetween. A shaft with a handle is attached to the takeup reel and extends out of the chamber to facilitate rotation of the takeup reel whereby the test sample may be removed from the supply reel to the testing station and then to the takeup reel. First and second electrodes are connected, respectively, to the high and low voltage terminals of a power supply. Position and pressure producing means are operatively connected to at least one of the electrodes to control the spacing between the electrodes and the pressure applied to the test sample by the electrodes.