Page images
PDF
EPUB

To find, therefore, the magnetic intensity which would produce the same effect on the orientation of the molecule as would be produced by rotating the body at the angular velocity N, all we have to do is to equate T and T. This gives

112
uH sin a ;
sin 8-Cul (1+ o)

2 w

scope when set into rotation. The first to see this, as well as the first to see any relation between magnetism and angular momentum, appears to have been Maxwell, who constructed apparatus for experiments on the subject as early as 1861.

In Maxwell's apparatus an electromagnet was pivoted in a circular frame in such a way as to be free to rotate about a horizontal line

COS

[blocks in formation]

(1+cos e) = 2.2 (1 + coso) through its center of mass and perpendicular

The values of 12 experimentally attainable are so small in comparison with any possible values of w that the last term is negligible. Hence we have for any molecule in the body, whatever its orientation and whether it contains one or more orbits,

m H = 232

to its magnetic axis. With the magnetic axis making an angle with the vertical, the frame was rotated at high speed about a vertical axis, also passing through the magnet's center of mass, and observations were made for a change in e, stability having been secured by suitable adjustments of the principal moments of inertia. No change could be detected, but only rough observations were possible.

In the experiments on magnetization by rotation Maxwell's electromagnet is replaced by each one of the countless multitude of molecular magnets of which the magnetic body is constituted, and the total change in the orientations of all these magnets with reference to the axis of rotation of the body is determined magnetically.3

[blocks in formation]
[ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]

m

H/N

= 47

e

FLUXMETER

[merged small][ocr errors][merged small][merged small][merged small][merged small]

If we assume that negative electrons alone are in orbital revolution, the value of the second member of this equation, according to well known experiments on electrons in slow motion, is - 7.1 x 10-7 electromagnetic units, and H/N should be equal to this quantity and identical for all magnetic substances. If positive electrons also participate the magnitude of H/N should be smaller.

If the Ampèreian currents consist in the motion of actual matter, so that the molecules of magnetic substances have angular momentum, an ordinary magnet or electromagnet itself should behave to some extent like a gyro

Two series of experiments have been made, both with Mrs. Barnett's assistance, and by methods as different from one another as possible. The first series of experiments was made

3 I have learned very recently from a footnote in John Perry's Spinning Tops that he made experiments on this subject, with the same idea in mind, but without success, many years ago.

on large iron rods by a method depending on in part of the work, and by an air turbine in
the principles of electromagnetic induction; the rest.
the second, on smaller rods of iron, cobalt and In making observations fluxmeter deflections
nickel, by the method of the magnetometer. were obtained for each of several speeds, first
Recently a few preliminary experiments have with the rotation in one direction and then
been made on Heusler alloy.

with the rotation in the other direction. Some of the essential parts of the apparatus After making a great variety of tests, and used in the first investigation are shown in after taking many precautions to eliminate the diagram of Fig. 3.

sources of error, two effects stood out very Two nearly similar rods of steel shafting A. clearly as the result of the observations, instead and B were mounted with their axes horizontal of the one which was looked for. and perpendicular to the magnetic meridian, If the mean of the two deflections for the and two similar coils of insulated copper wire same speed is plotted against the square of the were mounted about their centers. These coils speed, the resulting graph is a straight line as were connected in series with one another and shown in Fig. 4. The mean deflection is thus with a Grassot fluxmeter, which was the prin proportional to the square of the speed. This cipal measuring instrument, and were oppo- deflection is due to the increase of the residual sitely wound so that that any variations in the magnetic flux through the rotor produced by intensity of the earth's magnetic field acting its centrifugal expansion during rotation-an in the same way on both rods might produce effect which was not foreseen, and which was

[graphic][graphic][ocr errors][subsumed][subsumed][merged small][subsumed][merged small][merged small]
[ocr errors]

no effect on the fluxmeter. One of the rods, very puzzling until its explanation became
which will be called the compensator, as A, re- apparent. This effect would vanish if the rod
mained at rest; while the other, called the were completely demagnetized initially.
rotor, as B, was alternately rotated and If, however, the difference between the two
brought to rest, the change of flux being de- deflections for the two directions of rotation,
termined by the fluxmeter, which, together instead of the mean deflection, is plotted
with the other apparatus, was standardized by against the speed, and not against the square
proper subsidiary experiments. For use in of the speed, a straight line again results, as
these experiments the rods A and B were uni- shown in Fig. 5. This is the effect which was
formly wound with solenoids of insulated wire. under investigation. The straight line shows

To prevent possible disturbances arising that H is proportional to N, as predicted. from the presence of the earth's magnetic field, The earlier experiments by this method gave the rotor was surrounded by a large electric for H/N the mean value — 3.6 X 10-7 e.m.u.; coil which approximately neutralized the the later and more precise experiments gave earth's intensity in the region occupied by the - 3.1X 107 e.m.u., with an experimental error rotor.

for a set of four double deflections equal to The rotor was directly driven in either di- about 12 per cent. The graph of Fig. 5 is rection at will by an alternating current motor drawn for these observations, the dotted

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

and they have since published additional experiments. Very recently another investigation of this converse effect has been made by J. Q. Stewart. All these investigations are indirect but excellent confirmations of the work described here.

In the second investigation, as already stated, the method of the magnetometer was used. A diagram of important parts of the apparatus is given in Fig. 6.

The rod under test, or rotor, F, was mounted with its axis horizontal and normal to the magnetic meridian, as in the first investigation, and in the second, or equatorial, position of Gauss, which offered great advantages over the first, or polar, position for this work.

The magnetometer system, which was astatic, is shown suspended from the torsion head A by

compensating accurately the earth’s intensity with the large electric coil E, as in the earlier investigation. C is a small electric coil in series with E to make the zero and sensibility approximately independent of the compensating current in the coil E.

The rotors were driven by an alternating current motor, operating at the same speed in both directions. Three different speeds of the rotor could be obtained by using cone pulleys.

The principal magnetometer observations consisted in getting the double deflections produced by reversing the direction of the rotation, the speed for the two directions being the same. From these readings, the speed, and the calibration experiments, H, could be found as a

TABLE I

Rotor

Series

Groups

Mean
Speed
R.P.S.

H

Average
N

Depart-
Number

X 107

ure from
of Sets
E.M.U.

Mean
Mean

(Sets)

function of N. Numerous precautions were stances. There can be no question, however, necessary as in the earlier investigation. that the effect of the negative electrons is at

The results of the observations on four least greatly preponderant. rotors are given in Table I. The set” of A few preliminary results, not of a precise observations there referred to contained four character, but consistent with those of Table readings, or two double deflections.

I., have been obtained with a rotor of very soft With nickel and cobalt observations were iron and with a rotor of Heusler alloy-a made at more than one speed; and H/N was magnetic compound of aluminum, copper and found to be independent of the speed, within manganese in atomic proportions. the limits of the experimental error, as in the In summing up the chief results of the two earlier experiments with iron. It is also seen investigations it may be said that, in addition to be independent of the size and shape of the to revealing a second and entirely new method body in rotation, which is an implicit re of producing magnetization in magnetic subquirement of the theory developed above. stances, they have proved in a direct and con

clusive way, on the basis of classical dynamics

alone and without dependence on the still obIntrinsic Magnetic Intensity of Rotation in Iron,

scure theory of radiation, (1) that Ampèreian Nickel and Cobalt

currents, or molecular currents of electricity in orbital revolution, exist in iron, nickel, cobalt and Heusler alloy; (2) that all or most of the electricity in orbital revolution is nega

tive, or at least that the effect of the negative Steel

electricity is preponderant; and (3) that this (smaller) 1 1-2 44.8 21 5.1 0.5

electricity has mass or inertia. Furthermore, Steel

if we admit the classical theory of radiation, (larger) 2 3-4 47.8 21 5.2

according to which a ring of electrons moving Cobalt... 3 5–7 20.2 17

in a circular orbit must continually emit

4.8 2.2 4 8-11 30.3 23 5.6 1.2 energy, but at a smaller rate the more uni5 12-25 45.5 79 6.0 0.9

formly the electricity is distributed in the 6 22 45.0 7 6.5 0.3

ring, we must conclude that the electrons are 7

44.8

9 5.9 0.4 closely packed in the Ampèreian orbits. For 25 44.8 5 6.1 0.4

the existence of residual or permanent magneNickel ...

26 20.5 4 4.7 2.0 tization proves that these orbits are essentially 10 27-28 30.5 9 6.7 1.1

permanent and can not therefore emit energy 11 29-32 45.3 37 6.1 0.5

at an appreciable rate.

S. J. BARNETT The value of H/N is in all cases negative,

THE OHIO STATE UNIVERSITY but less in magnitude than that of the standard value of 4 7 m/e=—7.1 e.m.u. for negative electrons in slow motion, as was the case

THE ORIGIN OF THE PINK BOLLWORM in the earlier experiments with iron, which The determination of the original habitat of gave 3.6 and 3.1 in place of 7.1. In view of the pink bollworm (Pectinophora gossypiella the experimental errors, it still seems to me Saunders) is of great interest in relation to. doubtful whether these discrepancies indicate

the present distribution of this insect and may definitely that in addition to the negative elec be of importance later as indicating where trons in orbital revolution there are also posi- parasitic or other natural checks may be tive electrons revolving in orbits. The prob found. A scrutiny of the records gives strong ability of the presence of the latter orbits is

support to the theory that this insect origgreat from the known expulsion of a particles inated in Southern Asia, probably India. with great velocities from radio-active sub The first account of the insect by W. W.

1.2

24

Saunderg1 in 1842 accompanying its original description is based on specimens received from India, and the only information now available in relation to these specimens is an extract quoted by Saunders from a letter from a certain Dr. Barn, superintendent of the Government Cotton Plantations at Broach (Baruch) in western India. This extract is short and significant and is here given in full:

The inclosed is an insect which was very destructive to the American cotton which was sown here (Broach), on light alluvial soil. The egg is deposited in the germen at the time of flowering, and the larva feeds upon the cotton seed until the pod is about to burst, a little previous to which time it has opened a round hole in the side of the pod for air, and at which to make an exit at its own convenience, dropping on the ground, which it penetrates about an inch, and winds a thin web in which it remains during the aurelia state. Curiously enough, the cotton on the black soil was not touched by it. The native cotton is sometimes affected by it.

This letter was addressed to a certain Dr. Royle who forwarded the specimens with this quotation from Dr. Barn to Mr. W. W. Saunders. In relation to this quotation, Mr. Saunders makes this significant comment:

It is interesting to remark that the cotton grown from American seed is attacked in preference to any other and that the cotton plant .when grown upon black soil remains free from injury. The former fact could be accounted for by the American cotton being of a different species to that usually grown in India and probably offers seeds which are more suitable to development of the larvæ.

The reason for the greater susceptibility of and damage to the American cotton is undoubtedly that suggested by Mr. Saunders and is supported by many similar experiences with introduced plants or introduced plant pests. The hardy and rather unproductive cottons of India and other southern Asiatic countries probably long associated with this insect evidently were then and are still fairly resistant to its attacks, and, on the other hand, the introduced American and Egyptian varieties are

1 Saunders, W. W., Trans. Ent. Soc. London, Vol. III., pp. 284-85, 1843.

less resistant and perhaps furnish exceptional breeding conditions and were, therefore, when introduced into India and elsewhere in southern Asia, much more seriously attacked. This condition at once brought into prominence an insect which previously had been for the most part overlooked. It is significant that Dr. Barn should note that “native cotton is sometimes affected by it," indicating that it was a known but comparatively unimportant enemy of such cotton in India prior to 1842.

Saunders, in his article, makes no suggestion that the insect is other than a native Indian species, or, as has been stated by some writers, that it was imported with the American cotton. Responsibility for the theory of possible American at least African origin seems to rest with J. H. Durrant. This author, reviewing (1912)2 the specimens of Gelechia gossypiella in the British Museum, summarizes the earlier Indian records with an evident strong mental bias toward an inferred American or Egyptian origin. An examination of these records indicates that there is no real warrant for this bias. Of the Indian record of 1842, quoted above, from Saunders, he suggests the importation of the insect with American cotton simply because of the excessive damage to this introduced variety in comparison with native cottons, ignoring the perfectly reasonable explanation of this condition advanced by Saunders. The records for Cawnpore (1883) and Lahore (1893–94) report damage to “cotton” but this “cotton” is inferred by Durrant to be Egyptian because from other sources he learned that some Egyptian cotton was being experimentally grown at or near these places and, similarly, another record from Surat, about which no information was available, is assumed by Durrant to have a similar history.

August Busck (1917), following Durrant, without critical examination of the latter's data, accepts his general conclusions, and expresses the belief from this "evidence," and

2 Durrant, J. H., Bul. Ent. Research, Vol. III., Pt. 2, pp. 203-06, Fig. 1, London, 1912.

3 Busck, August, Jour. Agric. Research, U. S. D. A., Washington, Vol. IX., pp. 343–70, 6 pls., 1917.

« PreviousContinue »