Dr. Hertz, from his experiments on oscillating circuits, came to the conclusion that iron was not magnetic for very rapid frequencies; and, to quote from Fleming's abstract of Hertz's researches (vol. i., p. 416), “Hertz supposed that, as the self-inductance of iron wires is for slow alternations from eight to ten times that of copper wires, therefore a short iron wire would balance a long copper one; but this was not found to be the case, and he concludes that, owing to the great rapidity of the alternations, the magnetism of the iron is unable to follow them, and therefore has no effect on the self-induction.” And again, p. 423: “When the wire was surrounded by an iron tube, or when it was replaced by an iron wire, no perceptible effect was obtained, confirming the conclusion previously arrived at that the magnetism of the iron is unable to follow such rapid oscillations, and therefore exerts no appreciable effect.”

Steefan has, however, shown that we could expect very little alteration in the inductance of a wire, even if it were magnetic, on account of the greater concentration of the current in a magnetic conductor on the surface of the wire.

Professor J. J. Thomson (“Recent Researches,” p. 322, and “Philosophical Magazine,” 1891, p. 457) has shown that an iron cylinder placed in a solenoid absorbs considerably more energy than a similar non-magnetic conductor of equal conductivity, on account of the higher permeability of the iron.

J. Trowbridge (“Damping of Electric Oscillations”: Phil. Mag., December, 1891) has shown that the resistance of iron wires damps electrical energy very considerably, and has deduced that iron must have a fairly high permeability to account for the effects observed.

Lastly, we have the statement, in the last page of Gray's “Absolute Measurements,” that the damping of oscillations in a resonator is greater when the wire is of iron than when it is of copper.

In order to investigate the effect of “magnetic penetration” in iron for fields varying very much more rapidly than could be obtained with the use of the “time apparatus,” the readiest means to hand for obtaining a very rapid oscillatory current was the ordinary leyden-jar discharge.

The subject of the magnetization of iron in these fields has been very little touched upon since the time that Henry experimented on the effect of leyden-jar discharges on the magnetization of steel needles.

In the experiments that follow it will be shown that iron is strongly magnetic in rapidly-varying fields, even when the frequency is over 100,000,000 per second.

A solenoid was wound on a small glass tube, sixty turns of wire, seven turns to the centimetre. A leyden-jar, charged up to a convenient potential by a Voss machine, discharged through this solenoid, and any iron, whether solid or finely divided, placed inside the solenoid was always more or less magnetized by the discharge.

Plate XLVIII., Fig. 1.

C. ordinary leyden-jar; A is solenoid; S, spark-gap.

The whole of the discharge passed through solenoid A. After the discharge had passed the needles were examined by means of a small mirror magnetometer. As this magnetometer is used in all future experiments for testing the magnetization of needles, the construction is briefly explained. It was made on the pattern set forth in Gray's “Absolute Measurements,” vol. ii., p. 79. The needle was small, and arranged in a cavity, so that it was nearly dead-beat. The deflection was increased by means of a lamp and scale in the ordinary way. The value of the horizontal component at the needle was 0.22, and remained practically constant, as there were no masses of iron in the vicinity.

It was first settled that the needle placed in the solenoid was unaffected by the charging current from the Voss. The Voss was turned so as to charge up the jars just below the potential necessary to spark across knobs at S. The needle was then removed and tested by the magnetometer. No effect was observed.

The effect of discharges on needles of different diameters was first investigated. Length of needles, 7cm.:—

It will be observed from these experiments that the deflection is very nearly proportional to the diameter of the wire. This is to be expected, as the magnetizing forces are confined to a thin skin of the substance. The amount of the magnetization of the wire is proportional to the surface of the iron, and not to its sectional area, as it is for steady currents. In order to show that the effect was a surface one, and did not

penetrate any depth, a cylinder of thin copper was placed over the needle. The needle gave no appreciable deflection, showing that the copper cylinder completely screened off any effect on the iron. A thin external iron cylinder gave the same effect.

In order to determine with accuracy the state of a needle which had been under the influence of discharge, recourse was had to a method of solution of the iron. After several preliminary experiments, dilute HNO₃ at a temperature of boiling water was found to give the most reliable results. In order to test the rate at which the iron was eaten away a piece of pianoforte-wire 6.5cm. long, 0.032in. diameter, was taken and placed inside a solenoid, and subjected to a steady field of 100 C.G.S. units. The needle was then assumed to be magnetized uniformly throughout its section.

Plate XLVIII., Fig. 2.

E H F is a glass vessel, inside another glass vessel, A B C D, which is supported on a tripod of copper. Water is kept boiling in the outer vessel by a burner, K. Inside the inner vessel, but not touching it, is the needle, firmly fixed by the ends in a light frame. This frame is supported clear of the vessels by the stand, S.

The needle is fixed horizontally at a distance from the magnetometer, R, to give a convenient deflection on the scale. As the water is heated up to boiling-point the deflection due to the needle decreases slightly, due to the effect of temperature on the magnetic moment of the needle.

At a stand alongside, the dilute HNO₃ is kept in a beaker of boiling water, and when all is ready the HNO₃ is quickly transferred to the vessel E H F, taking care not to disturb the needle. The moment the HNO₃ reaches the level of the needle in the vessel the time is noted, for at that instant the needle commences to dissolve. Sufficient HNO₃ is poured in to cover the needle half an inch.

As the needle is dissolved the deflection falls, and the deflection at different intervals is carefully noted.

This method of fixing the needle first and then pouring in the acid was unavoidable, as the maximum deflection due to the needle could not otherwise be obtained. By keeping the HNO₃ at 100° C. and rapidly transferring it to the vessel (itself surrounded by boiling water) we insure that the needle is covered by HNO₃ at the same temperature during the whole time of solution. Since the amount of acid was large compared with the size of the needle, the effect of solution of the iron would not materially alter the rate at which the needle was dissolved.

A uniformly-magnetized steel needle was found to dissolve very regularly till it was reduced to an extremely fine filament, which did not break up until the magnetometer deflection had fallen within 3 div. of zero. The following is the result of an experiment on a uniformly-magnetized needle (needle 0.032in. in diameter; steady deflection just before acid is poured in = 222):—

It would be expected that the rate of solution of the metal at any instant would be ::^al to the surface of the metal at that instant—that is, to the radius of the wire. This is very accurately the case in the above experiment. The deflection of the magnetometer at any instant is proportional to the sectional area of the wire—i.e., to the square of the radius. The radius of the wire at any moment is therefore known.

If a curve is constructed whose abscissæ represent time, and ordinates the radii of the wire at different intervals, it will be found to be nearly a straight line, with the exception of an irregularity in the beginning of the curve.

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A needle of steel 0.032 in diameter was then taken, and magnetized by passing the discharge from four leyden-jars in parallel through the solenoid. Spark-length, 1/10in. A correction was made for the fall of deflection when needle was immersed in dilute HNO₃ at 100° C.

Plate XLVIII., Fig. 3.

The following is the table of observed values of time and deflection. The values of time and deflection are reduced for convenience in plotting curves:—

The steady deflection at first was 85. As the iron commenced to be eaten away the deflection rapidly rose, and reached its maximum, 156. It remained stationary for a short interval at its maximum value, and then rapidly decreased down to zero. When the deflection had fallen to zero the needle was removed, its diameter measured, and found to be 0.013in. The depth of magnetic penetration was therefore about 0.0095in.

Now, from the results of experiments on the eating-away of uniformly-magnetized needles, we see that the depth to which the iron is dissolved is proportional to the time. Since in 200sec. the depth dissolved was 0.0095in., the rate of solution = 0.000047in. per second.

If I represent intensity of magnetization of a thin circular shell, distance r from centre of the needle, and M the deflection of the magnetometer at any instant, —

Then ∫ I. 2πr. dr is ::^al to M;

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∴ I . r is ::^al to dM/dr.

If a be radius of needle at first, it has been shown that (a—r) is ::^al to t (the time of action of acid).

Let a—r = k. t,

then—dr = kdt,

and, substituting in equation (1), we get—

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I (a—kt) is ::^al to dM/dt, since dr is ::^al to dt;

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∴ I is ::^al to 1/a – kt . dM/dt

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Now, dM/dt is the value of the tangent of the angle that the tangent at any point of the curve B (see Plate XLVIII., Fig. 3) makes with the axis of abscissæ. dM/dt is ∴ known from curve B. We can consequently determine the curve of variation of I from the surface to the centre, although there are not sufficient data to actually calculate I in absolute measure.

Plate XLVIII., Fig. 4.

The curve in Fig. 4 is an approximate representation of the magnetization from the surface inwards. The ordinates represent I, the intensity of magnetization. The abscissæ represent the distances from the external surface of wire.

It will be observed that the surface-layer is magnetized in an opposite direction to the main part of the magnetized metal.

As we go inwards from the surface the intensity of magnetization rapidly decreases till at the point A there is a portion of the metal which is not magnetized. This will be called the “neutral point.”

On penetrating still further the magnetization changes sign, and rapidly rises to a maximum, which most probably represents an intensity corresponding to the saturation-point of steel. The intensity then remains practically constant till at D it decreases very rapidly down to zero.

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It is evident from the manner in which the magnetization varies inwards that the iron has been under the influence of an oscillatory discharge. The first half-oscillation penetrated to a depth of 1/100in., which is represented by the length O B in the figure. The neutral point A is at a depth of about 1/400in. from the surface. The second half-oscillation has evidently decreased in amplitude considerably, since the depth of penetration is only a quarter that of the first discharge.

In this experiment there is only evidence of two half-oscillations. Several needles were examined which had been magnetized under the influence of various fields and different lengths of spark-gap, but the existence of the return oscillation could not with certainty be detected.

All the needles used gave the same general result—viz., a thin surface-layer magnetized in one direction, and a thicker interior layer magnetized in the opposite direction.

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In one case examined the depth of penetration of the first discharge was considerably less than 1/1000in.

The effect of varying the capacity of the condenser and keeping the self-inductance and the spark-gap constant was also investigated.