The weight is only 2I 9 kilograms. A current of 25 amperes can be allowed to flow constantly without overheating, as the coils of the electromagnet, formed in flat layers with spaces between them, are cooled by an air blast furnished by a motor built into the iron circuit of the instrument. This motor takes 300 watts, while the electromagnet itself absorbs 3400 watts. A field strength of 48,000 gausses is produced over a circle IO millimetres in diameter, when the pole pieces are I millimetre apart. The author is justifiably proud that he has kept the rise of temperature in his field coils, 55 Β° C., within the limits of permissible heating set by German technicians, and, further, that by diminishing the diameter of his pole pieces, he can obtain as strong a field as du Bois got with an electromagnet weighing 65 per cent. more, and then only for a few minutes at a time. The famous magnet of Pierre Weiss, now Professor of Physics in the French University of Strasbourg, formerly at Zurich, had the disadvantage of requiring to be joined to a system of water supply in order to cool the coils. Its great weight was largely due to the oil surrounding the coils and to the water pipes in it. It took i8,ooo watts, but gave a stronger field by perhaps 20 or 3 Β° per cent. over a smaller area.
(3. F. S.