This paper contains an account of an electromagnetic method of testing for mechanical flaws in wire ropes such as colliery winding ropes, the method comprising the use of a “search band of flux ” in combination with a search-coil system.In the introduction are detailed the advantages of the use of alternating as compared with direct current for the excitation of the magnet system.Section 1 describes the results of an experimental investigation of the penetration of magnetic flux in “locked coil” wire ropes, for a supply frequency of 50 and 20 cycles per second respectively.In Section 2 the effect of mechanical strain on magnetic permeability is considered, with special reference to a.c. excitation.In Section 3 the change in reluctance of the air space due to eccentricity of the rope in the magnet system is examined.Section 4 contains an account of the construction of the rope-testing magnet system, and the method of utilizing the search band of flux is explained.Section 5 contains the results of tests on a length of wire rope which was specially constructed for the purpose by Messrs. Latch and Batchelor, and in which a series of hidden flaws were incorporated.A complete rope-testing equipment constructed in accordance with the principles explained in this paper has been installed in one of the Carlton Main Co.'s collieries in south Yorkshire.