The fixed and suspended coils of the dynamometer are respectively connected in series with the magnetizing solenoid and with a secondary wound upon the specimen.
This liability is overcome by making such movable parts as require to be magnetic of soft iron, and magnetizing them by the inducing action of a strong permanent magnet.
In the internal field of a long coil of wire carrying an electric current, the lines of force are, except near the ends, parallel to the axis of the coil, and it is chiefly for this reason that the field due to a coil is particularly well adapted for inductively magnetizing iron and steel.
The older operation of magnetizing a steel bar by drawing a magnetic pole along it merely consists in exposing successive portions of the bar to the action of the strong field near the pole.
The magnetic susceptibility expresses the numerical relation of the magnetization to the magnetizing force.
The action of a hollow magnetized shell on a point inside it is always opposed to that of the external magnetizing force, 6 the resultant interior field being therefore weaker than the field outside.
When either the magnetization I or the induction B corresponding to a given magnetizing force H is known, the other may be found by means of the formula B = 41rI + H.
K is a commutator for reversing the direction of the magnetizing current, and G a galvanometer for measuring it.
The strength of the magnetizing current is regulated by adjusting the position of the sliding contact E upon the resistance D.F.
The specimen upon which an experiment is to be made generally consists of a wire having a " dimensional ratio " of at least 300 or goo; its length should be rather less than that of the magnetizing coil, in order that the field Ho, to which it is subjected, may be approximately uniform from end to end.
Various currents are then passed through the magnetizing coil, the galvanometer readings and the simultaneous magnetometer deflections being noted.
Plate 59), the magnetizing field Ho being first gradually increased and then diminished to zero.
9) from F to D, while at the same time the commutator K is rapidly worked, a series of alternating currents of gradually diminishing strength being thus caused to pass through the magnetizing coil.
The ballistic method is largely employed for determining the relation of induction to magnetizing force in samples of the iron and steel used in the manufacture of electrical machinery, and especially for the observation of hysteresis effects.
The sample may have the form of a closed ring, upon which are wound the induction coil and another coil for taking the magnetizing current; or it may consist of a long straight rod or wire which can be slipped into a magnetizing coil such as is used in magnetometric experiments, the induction coil being wound upon the middle of the wire.
12 shows the nature of the course taken by the curve when the magnetizing current, after having been raised to the value corresponding to the point a, is diminished by steps until it is nothing, and then gradually increased in the reverse direction.
The downward course of the curve is, owing to hysteresis, strikingly different from its upward course, and when the magnetizing force has been reduced to zero, there is still remaining an induction of 7500 units.
Units in the case illustrated in the figure), an operation which is performed by simply reversing, the direction of the maximum magnetizing current a few times.
The sample under test is prepared in the form of a ring A, upon which are wound the induction and the magnetizing coils; the latter should be wound evenly over the whole ring, though for the sake of clearness only part of the winding is indicated in the diagram.
The magnetizing current, which is derived from the storage battery B, is regulated by the adjustable resistance R and measured by the galvanometer G.
By means of the three-way switch C the battery current may be sent either into the primary of E, for the purpose of calibrating the galvanometer, or into the magnetizing coil of the ring under test.
The reversing key K having been put over to the left side, the short-circuit key S is suddenly opened; this inserts the resistance R, which has been suitably adjusted before hand, and thus reduces the current and therefore the magnetizing force to a known value.
The experiment may be made in two different ways: (I) the magnetizing current is increased by a series of sudden steps, each of which produces a ballistic throw, the value of B after any one throw being proportional to the sum of that and all the previous throws; the magnetizing FIG.
When the magnetizing current is twice reversed, so as to complete a cycle, the sum of the two deflections, multiplied by a factor depending upon the sectional area of the specimen and upon the constants of the apparatus, gives the hysteresis for a complete cycle in ergs per cubic centimetre.
During the first stage, when the magnetizing force is small, the magnetization (or the induction) increases rather slowly with increasing force; this is well shown by the nickel curve in the diagram, but the effect would be no less conspicuous in the iron curve if the abscissae were plotted to a larger scale.
During the second stage small increments of magnetizing force are attended by relatively large increments of magnetization, as is indicated by the steep ascent of the curve.
Then the curve bends over, forming what is often called a " knee," and a third stage is entered upon, during which a considerable increase of magnetizing force has little further effect upon the magnetization.
Under increasing magnetizing forces, greatly exceeding those comprised within the limits of the diagram, the magAetization does practically reach a limit, the maximum value being attained with a magnetizing force of less than 2000 for wrought iron and nickel, and less than 4000 for cast iron and cobalt.
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).
The actual experiment to which it relates was carried only as the point marked X, corresponding to a magnetizing force of 65, and an induction of nearly 17,000.
[[[Magnetic Measurements]] that could be produced by any magnetizing force, however great.
It has, however, been shown that, if the magnetizing force is carried far enough, the curve always becomes convex to the axis instead of meeting it.
The full line shows the result of an experiment in which the magnetizing force was carried up to 585,1 FIG.
Trans., 1885, 176, 455) introduced a modification of the usual ballistic arrangement which presents the following advantages: (I) very considerable magnetizing forces can be applied with ordinary means; (2) the samples to be tested, having the form of cylindrical bars, are more easily prepared than rings or wires; (3) the actual induction at any time can be measured, and not only changes of induction.
Thick, through which is cut a rectangular opening to receive the two magnetizing coils B B.
Between the magnetizing coils is a small induction coil D, which is connected with a ballistic galvanometer.
Ewing (Magnetic Induction, § 194) has devised an arrangement in which two similar test bars are placed side by side; each bar is surrounded by a magnetizing coil, the two coils being connected to give opposite directions of magnetization, and each pair of ends is connected by a short massive block of soft iron having holes bored through it to fit the bars, which are clamped in position by set-screws.
With this arrangement it is possible to find the actual value of the magnetizing force, corrected for the effects of joints and other sources of error.
Between them and of the magnetizing coils being reduced to one-half.
If H l and H2 be the values of 47rinll and 47ri' - 'Z/ l for the 2 2 same induction B, it can be shown that the true magnetizing force is H = H l - (H 2 - H 1).
If a transverse cut is made through a bar whose magnetization is I and the two ends are placed in contact, it can be shown that this force is 27r I 2 dynes per unit of area (Mascart and Joubert, Electricity and Magnetism, § 322; and if the magnetization of the bar is due to an external field H produced by a magnetizing coil or otherwise, there is an additional force equal to HI.
Thus the whole force, when the two portions of the bar are surrounded by a loosely-fitting magnetizing coil, is.
If each portion of the bar has an independent magnetizing coil wound tightly upon it, we have further to take into account the force due to, the mutual action of the two magnetizing coils, which assists.
It is, of course, true for permanent magnets, where H = o, since then F = 27rI 2; but if the magnetization is due to electric currents, the formula is only applicable in the special case when the mutual action of the two magnets upon one another is supplemented by the electromagnetic attraction between separate magnetizing coils rigidly attached to them.2 The traction method was first employed by S.
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.
Instead of a divided ring he employed a divided straight bar, each half of which was provided with a magnetizing coil.
The sample has the form of a thin rod, one end of which is faced true; it is slipped into the magnetizing coil from above, and when the current is turned on its smooth end adheres tightly to the surface of the yoke.
The instrument exhibited by Thompson would, without undue heating, take a current of 30 amperes, which was sufficient to produce a magnetizing force of woo units.
The test-piece A, surrounded by a magnetizing coil, is clamped between two soft-iron blocks B, B'.
The actual magnetizing force H is of course less than that due to the coil; the corrections required are effected automatically by the use of a set of demagnetization lines drawn on a sheet of celluloid which is supplied with the instrument.
1898, 27, 526), the value of the magnetic induction corresponding to a single stated magnetizing force is directly read off on a divided scale.
The standard rod and the test specimen, which must be of the same dimensions, are placed side by side within two magnetizing coils, and each pair of adjacent ends is joined by a short rectangular block or " yoke " of soft iron.
For simplicity of calculation, the clear length of each rod between the yokes is made 12.56 (=47r) centimetres, while the coil surrounding the standard bar contains 100 turns; hence the magnetizing force due to a current of n amperes will be ion C.G.S.
Suppose the switches to be adjusted so that the effective number of turns in the variable coil is loo; the magnetizing forces in the two coils will then be equal, and if the test rod is of the same quality as the standard, the flow of induction will be confined entirely to the iron circuit, the two yokes will be at the same magnetic potential, and the compass needle will not be affected.
But a balance may still be obtained by altering the effective number of turns in the test coil, and thus increasing or decreasing the magnetizing force acting on the test rod, till the induction in the two rods is the same, a condition which is fulfilled when reversal of the current has no effect on the compass needle.
Let m be the number of turns in use, and H 1 and H2 the magnetizing forces which produce the same induction B in the test and the standard rods respectively; then H1=H2Xm/Ioo.
But when exceptionally strong fields are desired, the use of a coil is limited by the heating effect of the magnetizing current, the quantity of heat generated per unit of time in a coil of given dimensions increasing as the square of the magnetic field produced in its interior.
These results are of extreme interest, for they show' that under sufficiently strong magnetizing forces the intensity of magnetization I reaches a maximum value, as required by W.
C is a " compensating coil " consisting of a few turns of wire through which the magnetizing current passes; it serves to neutralize the effect produced upon the magnetometer by the magnetizing coil, and its distance from the magnetometer is so adjusted that when the circuit is closed, no ferromagnetic metal being inside the magnetizing coil, the ti, magnetometer needle undergoes no deflection.
When a hysteresis curve is to be obtained, the procedure is as follows: The current is first adjusted by means of R to such a strength as will fit it to produce the greatest + and - values of the magnetizing force which it is intended to apply in the course of the cycle; then it is reversed several times, and when the range of the galvanometer throws has become constant, half the extent of an excursion indicates the induction corresponding to the extreme value of H, and gives the point a in the curve fig.