Improvements in Apparatus for Testing Magnetic Quality.
Year: 1896

page_text: Date of Application, 25th Jan., 1896
Complete Specification Left, 10th Oct., 1896-Accepted, 28th Nov., 1896
PROVISIONAL SPECIFICATION.
Improvements in Apparatus for Testing Magnetic Quality.
I, JAMES ALFRED EWING of Langdale Lodge Cambridge, Professor of Mechanism
in the University of 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 the relation
of magnetising force to magnetic induction in specimens of iron steel or other
magnetic metals. I test the specimen in the form of a cylindrical rod by
comparing it with a similar standard rod the magnetic quality of which has been
examined once for all. The two rods are connected by means of iron yokes at
their ends so that they form two limbs of a single magnetic circuit. Magnetising
forces are applied to both by means of coils wound upon them and the test is
made by comparing the magnetising forces which produce the same magnetic
induction in the two. I vary the number of ampère-turns on one rod relatively
to those on the other in order to find the relative amounts of magnetising force
which are required to produce equal magnetic inductions. This is most conveniently
done by varying the number of turns used in the magnetising coil on one of the rods,
the number of turns in the other coil being constant, while the same current
traverses both coils in series. I use switches to change the number of turns acting
on one rod, and these are further so arranged that when any number of turns is
thrown in or cut out a corresponding resistance is automatically cut out of or thrown
into the circuit with the effect that the manipulation of the switches does not affect
the strength of the current. The current passing through both coils in series is
brought to any desired strength by a rheostat or otherwise and its amount is
observed by means of an ampère-meter. The number of effective turns in the
coil surrounding the rod to be tested is varied until the yokes at the ends of the
rods are brought to the same magnetic potential, which state of balance corresponds
with equal inductions in both rods. To detect any difference in magnetic potential
between the two end-yokes I use a cross-yoke which forms an easy path for
magnetic induction between one end-yoke and the other. When the end yokes are
at the same magnetic potential they will produce no induction in the cross-yoke.
Various forms of magnetic detector may be used to test whether there is induction
in the cross-yoke: a good arrangement is to make a gap in the cross-yoke and have
a compass needle in the gap. On reversing the magnetising current the detector
shows that there is no magnetic induction in the cross-yoke provided the induction
is the same in both rods. The relative magnetising forces are then given by the
relative numbers of ampère-turns, and the value of the induction is inferred from
the previously known relation of magnetising force to induction in the standard rod.
Dated this 21th day of January 1896.

page_text: Improvements in Apparatus for Testing Magnetic Quality.
I, 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 the relation
of magnetising force to magnetic induction in specimens of iron steel or other
magnetic metals. I test the specimen in the form of a cylindrical rod by comparing
it with a similar standard rod the magnetic quality of which has been examined
once for all. The two rods are connected by means of iron yokes at their ends
so that they form the limbs of a single magnetic circuit. Magnetising forces
are applied to both by means of coils wound upon them, and the test is made by
comparing the magnetising forces which produce the same magnetic induction in
the two.
I vary the number of ampere-turns on one rod relatively to the number
on the other in order to find the relative amounts of magnetising force which are
required to produce equal magnetic inductions. This is most conveniently done by.
varying the number of tarns used in the magnetising coil on one of the rods,
the number of turns in the other coil being constant, while the same current traverses
both coils in series. I use switches to change the number of turns acting on
one rod, and these switches are further so arranged that when any turns are
thrown in or cut out a corresponding resistance is automatically cut out of or
thrown into the circuit, with the effect that the manipulation of the switches
does not affect the strength of the current. The current passing through both
coils in series is brought to any desired strength by means of a rheostat or
otherwise, and its amount is observed by means of an ampere-meter. The
number of effective turns in the coil surrounding the rod to be tested is varied
until the yokes at the ends of the rods are brought to the same magnetic potential,
which state of balance corresponds with equal inductions in both rods. To detect
any difference in magnetic potential between the two end-yokes I use a cross-yoke
which forms an easy path for magnetic induction between one end-yoke and the
other. When the end-yokes are at the same magnetic potential they will produce
no induction in the cross-yoke. Various forms of magnetic detector may be used
to test whether there is induction in the cross-yoke; a good arrangement is to
make a gap in the cross-yoke and to have a compass-needle in the gap. On
reversing the magnetising current the detector shows that there is no magnetic
induction in the cross-yoke provided the induction is the same in both rods.
The relative magnetising forces are then given by the relative numbers of
ampere-turns, and the value of the induction is inferred from the previously known
relation of magnetising force to induction in the standard rod. The current may
be adjusted to produce any desired magnetising force or any desired induction in
the standard rod. By using a series of different currents complete data are
obtained of the relationship of induction to magnetising force in the rod under
examination.
In Figures 1 and 2, which illustrate one form of the apparatus, A is the rod
to be tested and A' is the standard red. The ends of the rods are clamped in the
iron yokes B B which in this example are held apart by brass stays FF F. Two
iron pieces GG are connected to the yokes B B, forming what I have termed a
cross-yoke. The cross-yoke is interrupted by a gap between the ends of G G in
which the magnetic detector H is placed, consisting of a short compass-needle with a
long pointer, pivoted in a box and controlled by an adjustable directing magnet K.
The rods A A slip within bobbins which are wound with magnetising coils, and one

page_text: of these coils, namely that on A, is divided into sections and the ends of these sections
are connected to switches in such a manner that any number of turns up to the full
number of 200 or more may be put into action. I generally use 100 turns for the
magnetising coil on the standard rod A', and make the clear length of each rod equal
to 12.56 centimetres, which causes one ampere to produce ten units of magnetising
force in the standard rod. The switches by which the number of effective turns
acting on A is varied have double contacts and maintain the total resistance of the
circuit unaltered when coils are switched in or out by introducing compensating
resistances. The effective coils on A and the coils on A¹ are connected in series in such a
direction that the lines of magnetic induction passing from right to left along A return
from left to right along A'. The number of effective turns on A is adjusted until the
yokes BB are found to have the same magnetic potential, which is shown by the absence
of deflection in the magnetic detector H. This condition is most conveniently tested by
repeatedly reversing the current, and a reversing key is provided for that purpose.
In testing sheet metal the rod is made up of any convenient number of strips and in
that case the standard rod may similarly be made up of strips.
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 the magnetic quality of iron or other magnetic metals in
which a rod of the metal to be tested is compared with a standard rod by arranging
the two rods with yokes and magnetising coils to form a complete magnetic circuit
and observing the relative number of ampere-turns required to bring the two yokes to
the same magnetic potential.
(2.) The combination, for the purpose of magnetically testing iron or other metals,
of two rods wound with magnetising coils one or both of which has a variable number
of turns, with yokes connecting the ends of the rods and a magnetic detector between
the yokes.
(3.) The apparatus for testing magnetic quality in iron steel or other magnetic
metals substantially as described.
Dated thin 9th low of
aha 1000