The first exact experiments demonstrating the changes which occur in the permeability of iron,, 3 Phil.
The practice of measuring magnetic induction and permeability with scientific accuracy was introduced in 1873 by H.
So far, the best results have been attained with aluminium, and the permeability was greatest when the percentages of manganese and aluminium were approximately proportional to the atomic weights of the two metals.
In regard to water, all soils have two actions - namely, permeability and absorbability.
Permeability and Susceptibility.
The specimens distinguished by unusually high permeability were constituted as follows: Silicon-iron.
Above these temperatures the little permeability that remained was found to be independent of the magnetizing force, but it /1, appeared to vary a little with the temperature, one specimen showing a permeability of 100 at 820°, 2.3 at 950°, and 17 at 1050°.
The silicon-iron had, in fields up to about Io, a greater permeability than a sample of the best Swedish charcoal-iron, and its hysteresisloss for max.
- The ratio B/H is called the permeability of the medium in which the induction is taking place, and is denoted byµ.
(26) Also A = B = H H4?rI _ I +41K, (27) and (28) 471 Since in empty space B has been assumed to be numerically equal to H, it follows that the permeability of a vacuum is equal to i.
In the case of the ferromagnetic metals and some of their alloys and compounds, the permeability has generally a much higher value.
No substance has yet been discovered having a negative susceptibility sufficiently great to render the permeability (= I +471K) negative.
The total magnetic induction or flux corresponds to the current of electricity (practically measured in amperes); the induction or flux density B to the density of the current (number of amperes to the square centimetre of section); the magnetic permeability to the specific electric conductivity; and the line integral of the magnetic force, sometimes called the magnetomotive force, to the electro-motive force in the circuit.
The principal points of difference are that (I) the magnetic permeability, unlike the electric conductivity, which is independent of the strength of the current, is not in general constant; (2) there is no perfect insulator for magnetic induction, which will pass more or less freely through all known substances.
The presence of ordinary impurities usually tends to diminish the permeability, though, as will appear later, the addition of small quantities of certain other substances is sometimes advantageous.
When it is mechanically hardened by hammering, rolling or wire-drawing its permeability may be greatly diminished, especially under a moderate magnetizing force.
An experiment by Ewing showed that by the operation of stretching an annealed iron wire beyond the limits of elasticity the permeability under a magnetizing force of about 3 units was reduced by as much as 75%.
The permeability of a soft iron wire, which was tapped while subjected to a very small magnetizing force, rose to the enormous value of about 80,000 (Magnetic Induction, § 85).
Curves of Permeability and Susceptibility.-The relations of µ (= B/H) to B, and of to I may be instructively exhibited by means of curves, a method first employed by H.
If, however, the permeability of the test rod differs from that of the standard, the number of lines of induction flowing in opposite directions through the two rods will differ, and the excess will flow from one yoke to the other, partly through the air, and partly along the path provided by the bent bars, deflecting the compass needle.
24 was converted into an almost perfectly straight line passing through the origin, and lying below the horizontal axis; while the permeability of the metal was greatly diminished by the operation.
The following are the chief results of Hopkinson's experiments: For small magnetizing forces the magnetization of iron steadily increases with rise of temperature till the critical temperature is approached, when the rate of increase becomes very high, the permeability in some cases attaining a value of about i i,000; the magnetization then with remarkable suddenness almost entirely disappears, the permeability falling to about 1.14.
Steel behaves in a similar manner, but the maximum permeability is not so high as in iron, and the fall, when the critical point is approached, is less abrupt.
After one of the rings had been annealed at 840°, its maximum permeability at ordinary temperatures was 4000 for H =1.84; when it had been subsequently annealed at 1150°, the maximum permeability rose to 4680 for H =1.48, while the hysteresis loss for 2 B = t 4000 was under 500 ergs per c.cm.
Specimens of curves showing the relation of induction to magnetic field at various temperatures, and of permeability to temperature with fields of different intensities, are given in figs.
The behaviour of cobalt is particularly noticeable; its permeability increased with rising temperature up to a maximum at 500°, when it was about twice as great as at ordinary temperatures, while at 1600°, corresponding to white heat, there was still some magnetization remaining.
They showed that the permeability of this sample of iron was considerably diminished at the lower temperature.
The maximum permeability (for H = 2) was 3400 at 15° and only 2700 at - 186°, a reduction of more than 20%; but the percentage reduction became less as the magnetizing force departed from the value corresponding to maximum permeability.
Observations were also made of the changes of permeability which took place as the temperature of the sample slowly rose from - 186° to 15°, the magnetizing force being kept constant throughout an experiment.
The values of the permeability corresponding to the highest and lowest temperatures are given in the following table.
Most of the permeability-temperature curves were more or less convex towards the axis of temperature, and in all the experiments except those with annealed iron and steel wire, the permeability was greatest at the lowest temperature.
They found that the permeability of Swedish iron, tungsten-steel and nickel, when the metals were cooled to - 186°, was diminished in weak fields but increased in strong ones, the field in which the effect of cooling changed its sign being 115 for iron and steel and 580 for nickel.
The permeability of cobalt, both annealed and unannealed, was always diminished at the low temperature.
Claude (C. R., 1899, 129, 409) found that for considerable inductions (B =15,000) the permeability and hysteresis-loss remained nearly constant down to - 186°; for weak inductions both notably diminished with temperature.
A sample of Hadfield's manufacture, containing 1 2.36% of manganese, differed hardly at all from a non-magnetic substance, its permeability being only 1.27.
The permeability of the alloys containing from 1 to 4.7% of nickel, though less than that of good soft iron for magnetizing forces up to about 20 or 30, was greater for higher forces, the induction reached in a field of 240 being nearly 21,700.
The induction for considerable forces was found to be greater in a steel containing 73% of nickel than in one with only 33%, though the permeability of pure nickel is much less than that of iron.
The addition of silicon in small quantities considerably diminished permeability and increased coercive force; but when the proportion amounted to 2.5% the maximum permeability (µ =5100 for H =2) was greater than that of the nearly pure iron used for comparison, while the coercive force was only 0.9.
6 A small percentage of aluminium produced still higher permeability (µ=6000 for H=2), the induction in fields up to 60 being greater than in any other known substance, and the hysteresis-loss for moderate limits of B far less than in the purest commercial iron.
The aluminium-iron attained its greatest permeability in a field of o 5, about that of the earth's force, when its value was 9000, this being more than twice the maximum permeability of the Swedish iron.
For additional information regarding the composition and qualities of permanent magnet steels reference may be made 6 The marked effect of silicon in increasing the permeability of cast iron has also been noticed by F.
When the proportion of aluminium to manganese was made a little greater or smaller, the permeability was diminished.
Rowland,' whose careful experiments led to general recognition of the fact previously ignored by nearly all investigators, that magnetic susceptibility and permeability are by no means constants (at least in the case of the ferromagnetic metals) but functions of the magnetizing force.
It varies with the increase of the intracapillary or decrease of the extracapillary pressure, and is also in part regulated by the greater or lesser permeability of the vessel-walls.
Thus, while increased pressure in the blood or lymph vessels may be one factor, and increased permeability of the capillary endothelium another, increased osmotic pressure in the tissues and lymph is probably the most important in the production of dropsy.
Permeability is practically identical with the speed at which percolation takes place; through clay it is slow, but increases in rapidity through marls, loams, limestones, chalks, coarse gravels and fine sands, reaching a maximum in soil saturated with moisture.
On the other hand, the same observations go to show that the disease is met with oftener on the more recent formations than the older, and this fact, so far as concerns the physical characters of the soil, is identical with the questions of permeability to air and water.
It has been variously attributed to metamorphism, consequent upon igneous intrusion, earth movements and other kinds of geothermic action, greater or less loss of volatile constituents during the period of coaly transformation, conditioned by differences of permeability in the enclosing rocks, which is greater for sandstones than for argillaceous strata, and other causes; but none of these appears to be applicable over more than limited areas.
Its physical properties, permeability by water, extensibility and elasticity, receive their interpretation in the needs of the latter.
The peculiarity of the protoplasm in almost every cell is that it is especially active in the regulation of its permeability by water.
The response to the stimulus takes the form of increasing the permeability of particular cells of the growing structures, and so modifying the degree of the turgidity that is the precursor of growth in them.
The extent of the area affected and of the variation in the turgor depends upon many circumstances, but we have no doubt that in the process of modifying its own permeability by some molecular change we have the counterpart of muscular contractibility.
Thus the scenery of a limestone country depends on the solubility and permeability of the rocks, leading to the typical Karst-formations of caverns, swallowholes and underground stream courses, with the contingent phenomena of dry valleys and natural bridges.
The permeability of most material substances differs very slightly from unity, being a little greater than I in paramagnetic and a little less in diamagnetic substances.
If a hollow sphere 7 of which the outer radius is R and the inner radius r is placed in a uniform field Ho, the field inside will also be uniform and in the same direction as Ho, and its value will be approximately 3 i - R 3 For a cylinder placed with its axis at right angles to the lines of force, 2 = Ho (41) 2 +4(-2)(i - R2) These expressions show that the thicker the screen and the greater its permeability o, the more effectual will be the shielding action.
To secure the highest possible permeability it is essential that the iron should be softened by careful annealing.
But though the force was thus increased ninefold, the induction only reached 19,800, and the ultimate value of the permeability was still as much as 33'9.
Below is given a selection from Bidwell's tables, showing corresponding values of magnetizing force, weight supported, magnetization, induction, susceptibility and permeability: - A few months later R.
An excellent instrument of the class is Ewing's permeability bridge.
For strong magnetizing forces (which in these experiments did not exceed II= 48.9) the permeability remains almost constant at its initial value (about 400), until the temperature is within nearly i oo of the critical point; then the permeability diminishes more and more rapidly until the critical point is reached and the magnetization vanishes.
The most striking feature presented by these is the enormous value, 12,660, which, with H =0.153, is, attained by the permeability at 765° C., followed by a drop so precipitous that when the temperature is only 15° higher, the value of the permeability has become quite insignificant.
Soc., 1896, 60, 81) were the first to experiment on the permeability and hysteresis of iron at low temperatures down to that of liquid air (-186° C.).
Coupling together these ideas he was finally enabled to prove that the propagation of electric and magnetic force takes place through space with a certain velocity determined by the dielectric constant and the magnetic permeability of the medium.
If we imagine the current in the conductor to be instantaneously reversed in direction, the magnetic force surrounding it would not be instantly reversed everywhere in direction, but the reversal would be propagated outwards through space with a certain velocity which Maxwell showed was inversely as the square root of the product of the magnetic permeability and the dielectric constant or specific inductive capacity of the medium.