This paper deals with the details of design and operation of air core water‐cooled coils dissipating up to 1700 kilowatts. Four magnets have been built and used. The first is intended primarily for the measurement of the magnetization of any substance at any temperature in fields up to 40,000 gauss. The second was designed to test the desirability of a new type of construction. The third gives fields up to 100,000 gauss uniform within 1 percent in a volume of 25 cc, and is to be used primarily for Zeeman effect and other optical experiments. The fourth can produce fields up to 50,000 gauss uniform within 1 percent in a volume of almost a liter, and is designed primarily for experiments at very low temperatures. The power to operate these magnets is supplied by a motor generator capable of giving 10,000 amperes at 170 volts continuously.