PERMEAMETER, an instrument for measuring the permea bility of a sample of iron or steel, and more generally for exam ining the magnetic characteristics. The name was first applied by S. P. Thomson to an apparatus devised by himself in 189o. In this the sample, in the form of a rod, is surrounded by a mag netizing coil, and the mechanical force required to detach one end from an iron yoke of special form is measured by a spring balance. The force varies as the square of the induction, so that the induction, and hence the permeability for a known magnetizing field can be calculated. The measurement of the tractive force is also the basis of the du Bois magnetic balance permeameter.
In commercial routine magnetic testing measurements of a number of corresponding values of the induction, B, and of the magnetic field, H, are required. The Thomson and du Bois per meameters are rapid, and have been much used, but they are unsatisfactory for accurate work. The most accurate method of magnetic testing is the ballistic method using ring-shaped speci mens. It is, however, desirable to carry out tests with ordinary bars, and a number of permeameters have been devised in which the same general method is extended to bar shaped specimens. The induction through a bar can be readily measured in the usual way by surrounding it by a search coil connected to a ballistic galvanometer (or fluxmeter) ; but the calculation of the mag netizing field is usually a matter of difficulty owing to the de magnetizing effects of the ends of the bar. By clamping the bar in
a massive yoke of iron, a condition of approximate endlessness is attained, the bar and yoke together forming a magnetic circuit.
This is done in the pioneer bar and yoke method of Hopkinson. In the crude arrangement, the joints (between the bar and yoke) and the yoke itself introduce great uncertainties. In Ewing's double bar and yoke test these errors are eliminated by a method involv ing a double series of measurements with two lengths of the bars. (See MAGNETISM.) One of the most accurate permeameters is the double bar permeameter of C. W. Burrows. In this, two approximately equal bars are clamped in yokes at their ends. The bars are surrounded by magnetizing coils, and on reversing the magnetizing current, by means of a series of search coils connected to a galvanometer, a test can be made as to whether the induction remains uniform along the bar under test. There will generally be variations along the bar owing to the action of the joints and yokes; hut this may be compensated by adjusting the current in subsidiary magnetizing coils. When uniformity of induction along the bar is attained, the magnetic field is that corresponding to the current in the main magnetizing coil, so that the field corresponding to the measured induction is accurately known.
For details of this permeameter, and of others, such as that of F. P. Fahy, which have come into general use, the technical literature should be consulted.