Electrical resistance heating to effect thermal fragmentation of semiconductive rock was examined by the Bureau of Mines. Tests were carried out on copper-bearing ores of the nonesuch shale formation in the laboratory and in the field at White Pine, Michigan. Fieldwork included primary and secondary breakage tests. Heating tests on in situ rock failed to produce thermal damage to an extent considered useful; however, boulders up to 30 ft3 were effectively broken by the method. Power input to the test rock was taken from a 50-kv, 5- amp DC power supply.