 The Kerr force device contains water. The tube has been partially evacuated. We will cool the bottom bulb in liquid nitrogen. As the bottom bulb is cooled, the high energy molecules which escape from the liquid in the top bulb are condensed to solid water in the bottom bulb. As the high energy molecules are removed from the water in the top bulb, the average kinetic energy of the water decreases. As the average kinetic energy of the water decreases, so does the temperature. Eventually, the average kinetic energy of the water molecules in the top bulb have been decreased enough so that the water in the top bulb solidifies.