The windings of the magnet are made of continuous lengths of copper tubing with cross section shown in Fig. 1. Water at high pressure is sent through the hollow center for cooling. The conductor is wound about the core in such a way that the resistance to the flow of the water is reduced to about half what would prevail in a magnet of conventional design. These features make possible a high effective current density. The disposition of the coils and the shape of the iron core were based on the experiences with the big magnets in Upsala and in Paris. A model gave the results indicated in Fig. 5. When the pole faces 2.5 cm in diameter were separated by 7 mm the field obtained was 33,200 gauss using 16 kw. The model was constructed to employ water at considerably higher pressures than were actually tried. A higher current density would then be permitted without risk of serious electrolytic corrosion.