P. Kapitza has recently described a method of producing magnetic fields of the order of half a million to a million gauss. The principle of the method is to pass an enormous amount of power into a solenoidal coil for a fraction of a second, the current being switched off from the coil before it has time to heat up seriously. Powers of 40,000 kilowatts are available for dissipation in the coil, and by this means magnetic fields of the order of half a million gauss are attainable. One of the most difficult problems of the work is to construct a satisfactory coil. When the current in the coil is 30,000 amperes and the magnetic field 500,000 gause, the electrodynamic forces on the strip of the coil are about 1·5 tons per centimetre length. It is quite easy, therefore, to attain pressures of the order of 7000 kgms. /sq. cm. in the body of the coil, forces which are far beyond the elastic limit of ordinary copper. It is necessary, therefore, to make a very careful study of the exact magnitude of the forces and resulting pressure distribution in the coil and to determine carefully the type of coil which will have the maximum strength. The second problem is to design the coil so as not to exceed the safe temperature of the insulation used and to produce at the same time the strongest magnetic field with the power available. This problem will be discussed first.