The transient containment of high magnetic energy densities in coils of various geometries is discussed. Inherent mechanical and thermal limitations are shown to apply to coils relying solely on the strength of materials. The usefulness of inertial effects is demonstrated. Instrumentation is described for producing 10 &mgr;sec range pulses of amplitude up to 1.6 megagauss in single‐turn coils. Experimental observations document the appearance of ``saw effect'' and Kruskal‐Schwarzschild instabilities at very high fields. The limitations characteristic of conventional coils can be overcome in force‐free geometries. The appropriate mathematics is developed and illustrated with practical examples.