By films of iron, nickel, and cobalt are here meant electrolytic deposits of these metals on platinised glass. The magnetic properties of such films are interesting in various ways. For example, when they are longitudinally magnetised, we have the case of a magnet where the ratio of length to breadth is very great, and where, consequently, the demagnetising force is very small, e.g., with a film of a 5000th part of a millimetre in thickness, and 5 mms. in length, we have a ratio in length to thickness of 25,000: 1; at the same time the film is so small that it can be tested in very high uniform fields.