Improvements in Apparatus for Testing Magnetic Quality in Iron.
Year: 1895

page_text: PROVISIONAL SPECIFICATION.
Improvements in Apparatus for Testing Magnetic Quality in Iron.
I, JAMES ALFRED EWING, Professor of Mechanism in the University of
Cambridge, of Langdale Lodge, Cambridge, do hereby declare the nature of this
invention to be as follows:-
The object of my invention is to provide an easy means of testing iron with
regard to the energy which is expended in reversing its magnetism, in consequence
of magnetic hysteresis. Such a test is specially important in sheet iron intended
for use in transformers or dynamo armatures, and my apparatus is designed to
allow specimens of sheet or other iron to be readily inserted and tested.
I cause
the specimen to suffer reversals of magnetism by turning it round, in a suitable
magnetic field, and I measure the work done upon it by reference to the mechanical
moment which is exerted between the specimen and the field. Or, alternatively,
I cause the magnetic field to turn and observe the moment which is required to
prevent the specimen from turning with it. In one arrangement of the apparatus
there is a permanent magnet supported on a knife-edge or otherwise so that it is
free to oscillate about a horizontal axis, and weighted so that it has some stability.
The specimen to be tested, which in the case of sheet iron may conveniently be a
strip or be made up of several strips cut to the same length and laid on one another,
is clamped in a holder which is made to revolve between the poles of the magnet,
about an axis in line with the axis of suspension of the magnet. The magnetism
of the specimen is reversed at each half-turn, and in consequence of the magnetic
hysteresis of the specimen the suspended magnet tends to follow it. The amount
of this tendency, which is proportional to the work expended in reversing the
magnetism, is measured either by noting the deflection of the magnet, by means of
a pointer and scale, or by applying a known mechanical moment to bring it back
to its normal position. The specimen may be in the form of a ring or rings instead
of strips.
In another arrangement the axis is vertical and the magnet is controlled by
means of a spring instead of by gravity.
Further the apparatus may be arranged so that the magnet is caused to revolve
while the specimen is left free to oscillate against the control furnished by a spring
or otherwise, and the work expended in reversing the magnetism is in that case
measured by observing how much the specimen tends to follow the revolving
magnet.
The magnetic circuit, which is made up of the magnet and the specimen with
the air gaps between them, is so proportioned that the amount of magnetic induction
in the specimen is only slightly affected by differences in magnetic permeability
between one specimen and another. Thus the induction is nearly constant so long
as specimens of the same length and section are used, or of the same length and
weight, and hence the work expended in magnetic reversals is fairly comparable
in different specimens. The scale may be calibrated by using weights to displace
the oscillating piece or by testing standard specimens whose magnetic quality is
known from independent experiments.
Dated this First day of February 1895

page_text: Improvements in Apparatus for Testing Magnetic Quality in Iron.
1, JAMES ALFRED EWING, of Langdale Lodge Cambridge, Professor of
Mechanism in the University of Cambridge do hereby declare the nature of this
invention and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
The object of my invention is to provide an easy means of testing iron with
regard to the energy which is expended in reversing its magnetism in consequence
of magnetic hysteresis. Such a test is specially important in sheet iron intended
for use in transformers or dynamo armatures, and my apparatus is designed to
allow specimens of sheet or other iron to be readily inserted and tested. I cause
the specimen to suffer reversals of magnetism by turning it round in a suitable
magnetic field, and I measure the work done upon it by reference to the mechanical
moment which is exerted between the specimen and the field. Or, alternatively,
I cause the magnetic field to turn and observe the moment which is required to
prevent the specimen from turning with it.
In one arrangement of the apparatus there is a permanent magnet supported on
a knife-edge or otherwise so that it is free to oscillate about a horizontal axis and
weighted so that it has some stability. The specimen to be tested, which in the
case of sheet iron may conveniently be a strip or be made up of several strips cut
to the same length and laid on one another, is clamped in a holder or carrier which
is made to revolve between the poles of the magnet about an axis in line with the
axis of suspension of the magnet. The magnetism of the specimen is reversed at
each half turn, and in consequence of the magnetic hysteresis of the specimen the
suspended magnet tends to follow it. The amount of this tendency, which is pro-
portional to the work expended in reversing the magnetism, is measured either by
noting the deflection of the magnet by means of a pointer and scale, or by applying
a known mechanical moment to bring it back to its normal position. The specimen
may be in the form of a ring or rings instead of strips.
In another arrangement the axis is vertical and the magnet is controlled by
means of a spring instead of by gravity.
Further, the apparatus may be arranged so that the magnet is caused to revolve
while the specimen is let free to oscillate against the control furnished by a spring
or otherwise, and the work expended in reversing the magnetism is in that case
measured by observing how much the specimen tends to follow the revolving
inaguet.
The magnetic circuit, which is made up of the magnet and the specimen with the
air gaps between them, is so proportioned that the amount of magnetic induction in
the specimen is only slightly affected by differences in magnetic permeability
between one specimen and another. Thus the induction is nearly constant so long
as specimens of the same length and section are used, or of the same length and
weight, and hence the work expended in magnetic reversals is fairly comparable in
different specimens. The scale may be calibrated by using weights to displace the
oscillating piece or by testing standard specimens whose magnetic quality is known
from independent experiments.
In order that my said invention may be most fully understood and readily
carried into effect I will proceed to describe the drawings hereto annexed, which
show one form of the apparatus in elevation and plan. In this case the specimen
of iron to be tested is caused to revolve and the field-magnet is free to oscillate
about a horizontal axis. The iron to be tested is cut or stamped in the form of
strips of a standard length (which in this example is three inches). The number
of strips taken to compose the sample depends on the thickness of the sheet; six or
seven will in general be sufficient with transformer iron of usual thickness; and

page_text: such a number is taken that the whole sample approximates to a standard weight..
The sample is placed in a carrier A and is covered with a vulcanite washer aud
secured by clamps B B. The carrier is made to revolve by means of the friction
wheel C and driving wheel D. The sample revolves between the poles of a
permanent field-magnet E with the effect that the magnetism of the sample is.
reversed at each half turn. The work done in reversing the magnetism, in
consequence of bysteresis, causes a mechanical moment to be exerted by the
revolving sample upon the magnet, and the magnet being supported on a knife-
edge at F in line with the axis of the carrier is deflected to an extent which serves
to measure the work expended. The deflection is observed by means of a pointer-
and scale. The swinging of the magnet is checked by means of a dash-pot I
consisting of a vane moving in a box filled with oil. The stability of the magnet
is adjusted to give any required degree of sensitiveness by means of the weight G
which travels as a nut upon a screw fixed to the magnet. The knife-edge F works
in an agate trough, and a lifting arrangement L is provided, operated by the
handle H to save the knife-edge from unnecessary wear or injury. The pointer-
may be set to a zero in the middle of the scale by means of a weight in the form of
a nut K which works on a screw projecting sideways from the middle of the-
magnet. In operating, the observer inserts the sample and secures it by the
clamps, then begins to turn the driving wheel and lets the magnet down on its-
knife-edge. After reading the deflection of the pointer to one side he may reverse
the direction of rotation and read the deflection to the other side. To interpret
the observed deflections precisely I prefer to supply with the apparatus two or
more standard samples the hysteresis of which has been determined. The
bysteresis of any other sample is readily found by comparing the deflection
produced by it with the deflections produced by the standard samples.
The dimensions and strength of the field-magnet may be proportioned with
reference to the section of the sample and the extent of the air gaps to give any-
desired value to the magnetic induction in the sample. An induction of about
4000 c.g.s. units will be found convenient. By increasing or reducing the area of
section of the sample the intensity of the induction is reduced or increased. A
considerable air space is left between the magnet poles and the ends of the sample
in order that such variations in magnetic permeability as are liable to be met with
in different samples may be almost without influence on the total induction through
the iron.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
(1.) Apparatus for testing samples of iron with respect to magnetic hysteresis
consisting of a carrier in which the sample is readily secured and a magnet which
induces magnetism in the sample, so arranged that the magnetism of the sample is.
reversed by rotating either the carrier or the magnet, together with an appliance
for indicating the mechanical moment which is exerted between the sample and the
magnet in consequence of this rotation.
(2.) Apparatus for testing samples of iron with respect to magnetic hysteresis.
consisting of a revolving carrier in which the sample is readily secured and a
magnet supported so as to be capable of deflection which induces magnetism in the
sample and is deflected in consequence of the hysteresis, together with a pointer or
other device showing the amount of this deflection.
(3.) The apparatus for testing magnetic hysteresis in iron substantially as.
described.
Dated this 95th tax of Oatcher 1905