Wirt Robinson.

The elements of electricity online

. (page 11 of 46)
Online LibraryWirt RobinsonThe elements of electricity → online text (page 11 of 46)
Font size
QR-code for this ebook

156. Magnetization Facilitated by Vibration. Vibration, how-
ever produced, is favorable to loosening up the molecules of a body.
Gilbert discovered that if an iron bar held in the magnetic merid-
ian be struck with a hammer it becomes a magnet, but no such
effect is produced if the bar be held crosswise.

The steel columns employed so largely in modern buildings are
all planted in the meridian and in accordance with the principle
stated in Par. 143 are penetrated lengthwise by the lines of force
of the earth's field (see Fig. 76). They are subjected to continual
vibration and therefore become in course of time highly mag-
netized. The lower ends of all such columns in the northern
hemisphere, being the ends from which the lines of force emerge,
are north poles (Par. 142). This also explains the fact which



several centuries ago caused great wonderment, that is, that iron
crosses on church steeples and the iron rods of weather vanes are
often found to have acquired magnetic properties.

Fig. 76.

157. Loss of Magnetization Facilitated by Vibration. Reflec-
tion will show that the foregoing principle works both ways, that
is, if an iron bar be placed in the meridian and jarred it acquires
magnetism; on the other hand, if a magnet not in the meridian
be jarred it loses its magnetism. Great care must then be observed
in handling magnets not to jar them by striking or by dropping
or otherwise, as under such conditions they deteriorate rapidly.
Even if a magnet be in the meridian when jarred, it loses strength
for the earth's field, being much weaker than the magnetic field
originally used in making the magnet, can not hold in position all
of the molecules when they begin to vibrate.

158. Effect of Heat. It is known that when a body is heated
its molecules are put into more or less violent vibration. When a
magnetic body is heated to a red heat the vibrations reach such a
pitch that they are no longer controlled by magnetic force, that
is, its molecules are dancing about so that the magnetic force can
no longer pull them into line, therefore, at a red heat magnets lose
their magnetism and magnetic bodies are no longer attracted and
can no longer be magnetized. If, however, such heated bodies be
allowed to cool in a magnetic field, the molecules as they quiet
down take positions in accordance with the magnetic force and the
result is that the bodies acquire magnetism. Gilbert found that
bars of iron or steel heated to redness and allowed to cool in the


meridian became magnets. If molten cast-iron be run into a
mould and cools and solidifies in a strong magnetic field it acquires

159. Magnetization Facilitated by Solution. When a body is
in solution it is separated into its individual molecules and these
have great freedom of movement. Deposition from solution
must take place molecule by molecule. Should a magnetic sub-
stance in solution be deposited while in a magnetic field, the
molecules should have no trouble in arranging themselves and the
resulting body, if our hypothesis be correct, should exhibit marked
magnetic properties. This has been confirmed experimentally by
depositing iron electrolytically (as electroplating is done) in a
strong field.

We are taught by geology that beds of iron ore are accumulated
through chemical processes by deposition from solution. Since
this deposition takes place in the earth's magnetic field, this
affords a reasonable explanation of the occurrence of the lode-
stone. Gilbert, although he wrote long before the atomic theory
had been advanced, evidently had this thought in mind and in
Chapter II, Book III of his work gives the following significant
experiment. "We once had chiselled and dug out of its vein a
lodestone twenty pounds in weight, having first noted and marked
its extremities; then after it had been taken out of the earth we
placed it on a float in water so it could freely turn about; straight-
way, that extremity of it which in the mine looked north turned to
the north in water and after a while there abode."




160. Most Suitable Metal for Making Magnets. We have
stated that soft iron is far more easily magnetized than steel but
on the other hand its retentivity, or power of retaining imparted
magnetism, is very slight. The best permanent magnets are made
from glass-hard steel, that is, steel which has been heated to a
bright red and then plunged into cold water. Certain metals
alloyed with steel improve its magnetic properties and others
injure or destroy them. An alloy of tungsten produces magnets
of great retentivity, while an alloy of manganese can hardly be
magnetized at all and has been proposed for structural work in
electrical laboratories.

161. Principle of Manufacture of Magnets. We have seen
that in theory a bar of steel becomes a magnet when its molecules
have been turned so that their poles lie in one direction. The
manufacture of magnets is based upon this theory, and just as we
get the individual hairs of a piece of fur to lie in one direction by
combing or by brushing or by blowing upon the fur, so we, in a
sense, comb or brush or blow the molecules of the bar of which
we wish to make a magnet. The principle of these processes is
the same; they differ merely in details of execution.

162. Magnetization by Single Touch. In this method the bar
to be magnetized is placed horizontal and preferably with its ends

Fig. 77.

resting upon or just in front of opposite poles of two magnets as
shown in Fig. 77. It may then be stroked from end to end with a
magnet, using the pole of the same kind as that near which the


last touched end of the bar is resting. A better way is to begin at
the middle of the bar and stroke it, the stroking magnet following
the path shown by the dotted line in the figure, then reverse ends
of the stroking magnet and stroke the other half of the bar in the
opposite direction. Finally, turn the bar over and repeat the
strokings upon the other side. The point last touched is of
opposite polarity to the pole with which it is stroked.

163. Magnetization by Divided Touch. This method is in
principle precisely the same as the foregoing and differs only in
that two magnets are used in the strokings. The opposite poles
of the two magnets are placed at the center of the bar (Fig. 78) and

Fig. 78.

are then drawn apart, the magnets following the paths shown by
the dotted lines in the figure. After eight or ten strokes, the bar
is turned over and the other side is stroked. Sometimes a block
of wood is placed between the poles of the stroking magnets and
they are held against this and slid back and forth along the bar,
being finally removed at the center of the bar. This last is called
the method by double touch.

164. Magnetization by an Electric Current. The best method
of magnetization is by means of an electric current. An insulated
wire is coiled around the bar to be magnetized and a current is


Fig. 79.

sent through the wire. As result of this treatment the bar be-
comes a magnet. A full explanation of this can not be given
without anticipating certain principles which have not yet been
developed and it must therefore be deferred until later, but it can
be stated that if a current flows through the coiled wire as shown
diagrammatically in Fig. 79 and in the direction of the small


arrowheads, there will be produced inside of the hollow of the coil a
magnetic field whose lines of force run as shown by the large arrow,
that is, inside of this hollow space around which the coil is wrapped
lines of force will run like a draught runs up a chimney and an
iron or steel bar placed in this field will have its molecules all
swept or "blown" into a common direction, and in the case of
steel a great part of them retain this new position. They really
turn in accordance with the principle given in the latter part of
Par. 144.

165. Consequent Poles. If a magnet be touched at some
point between its poles by a second magnet, the molecules around
that point may be disarranged to the extent of producing poles
intermediate to the original ones. Such intermediate poles are
called consequent poles. They are usually the result of some
accident or error in the process of magnetization. Their presence
may be detected by exploring the field about the magnet by means
of a small compass needle or by use of the magnetic figures. They
may be intentionally produced by stroking the bar in a different
manner from that prescribed or, in the electric method, by wrap-
ping a portion of the coil in opposite direction to the rest. If, for
example, a steel knitting needle be stroked with the north pole of
a magnet, the strokes beginning at each end and terminating at
the center, the needle will be found to have a north pole at each
end and a south pole at the center.

166. Magnetization Largely Confined to Outer Layers of
Magnet. The process of magnetization effects the outer layers
of the steel bar more than it does the interior portions. This may
be shown in several ways. If a magnet be placed in acid and its
outer layer be dissolved off, its magnetism will be found to de-
crease at a more rapid rate than its mass. Again, if a number of
thin flat pieces of steel, as for example blades of table knives, be
bound up in a bundle and magnetized, it will be found when the
bundle is taken apart that those on the outside of the bundle are
much more strongly magnetized than those on the interior. There
are three reasons for this. First, when the magnetism is imparted
by stroking, the outer layers act as a magnetic screen for the inner
layers (Par. 143), that is, the teeth of our magnetic comb do not
reach down into the deeper layers of molecules. Second, when the
electric method is used, the field is stronger close to the wire than


it is in the center of the coil so the outer portions of the bar become
more highly magnetized. Third, the outer layers act by induction
upon the inner layers and tend to produce in them an opposite
polarity, thereby weakening them. This last may be shown as
follows. If three similar steel bars be magnetized equally and then
tied together and used as a magnet, when they are again taken
apart the inner one will be found to be weaker than the outer ones.
Since thin ribbon-like bars can be more thoroughly magnetized
than thicker ones, the most powerful magnets are made of a

Fig. 80.

number of these separately magnetized and then bound together.
Such laminated magnets are powerful but, for reasons just explained,
their power does not increase in direct proportion to the number
of laminae or strips. It is found best to have the interior layers
project, as shown in Fig. 80, slightly beyond the outer layers.
Sometimes the ends of such magnets are inserted into soft iron
pole pieces.

167. Aging of Magnets. Even if they are handled carefully
and not jarred, magnets grow weaker with time, most probably on
account of the inductive effect mentioned in the preceding para-
graph, and may take several years to attain a constant state. In
certain electrical measuring instruments in which magnets are
used, it is of the utmost importance that these retain a constant
strength. Such magnets can be put through an artificial process
of aging by which the constant state can be reached quickly. This
treatment consists in exposing the newly made magnet to a current
of steam for some 20 hours, then remagnetizing it and exposing it
again to steam for ten hours.



168. Location of Earth's Magnetic Poles. We have seen that
Gilbert made the discovery that the earth itself is a magnet.
Starting from this point we are naturally led to enquire where are
its poles, what is the direction and intensity of its field at different
localities and, finally, why is it a magnet.

In experimenting with his spherical lodestones or terrellas Gil-
bert located their poles as follows. He laid a short piece of iron
wire upon the surface of the terrella near its equatorial region.
The wire became a magnet by induction and turning on the
polished surface of the sphere, as if on a pivot, pointed toward the
pole. The direction in which the wire pointed was marked with
chalk and the wire was then shifted to some other position and its
direction again marked. These chalk lines prolonged intersected
at the poles. Were the earth a homogeneous sphere its magnetic
poles could probably be located similarly, that is, the direction in
which a magnetic needle pointed at various localities could be
determined and these direction lines prolonged would intersect
at the poles, but the earth is far from being such a sphere and a
series of needles distributed around a parallel of latitude would
indicate directions not even approximately converging.

If we should start at any point with a magnetic needle and
move it continually in the direction of its length, just as is done
in the method described in Par. 141, we would not trace the arc of
a great circle but a curved line which if prolonged in both direc-
tions would eventually pass through the magnetic poles. Two
such lines would by their intersections locate the poles.

Figure 81 represents a portion of the northern hemisphere with
a series of such curves which begin at points along the equator
ten degrees apart. It will be noted that the north magnetic pole
does not coincide with the geographical pole and is in fact nearly
twenty degrees, or some twelve hundred miles, south of the latter.
It was discovered by Sir J. C. Ross during the arctic expedition
of 1829-33 and is located on the Island of Boothia Felix, north of



Hudson Bay, in latitude 70 5' north and longitude 96 43' west.
The south magnetic pole has not been reached. It is located in the
antarctic regions in approximately latitude 73 30' south and
longitude 147 30' east, whence it is seen that the two magnetic
poles are not at the extremities of a diameter of the earth.

It follows from the foregoing that what we have designated in
the preceding pages as the magnetic meridian, or the vertical plane
through the axis of the poised needle, does not in general pass
through the magnetic poles and furthermore changes its direction
from point to point.

169. Magnetic Declination. A study of Fig. 81 will show that
within its limits there are three and only three regions in which
the needle points to the geographic north pole. These regions,
marked A, B and C on the map, are the western side of Hudson
Bay, the vicinity of St. Petersburg in Russia and the eastern
portion of Siberia. At other points the needle points either to the
east or to the west of the true meridian. Thus, along a parallel
from St. Petersburg to Hudson Bay the needle points to the west
of the meridian, while continuing from Hudson Bay to Siberia it


points to the east. This deviation of the needle from the true
meridian is called the magnetic declination. We shall see later that
the declination at any locality is slowly changing. In 1905 along
a line through Charleston, S. C., Cincinnati, Ohio, Lansing,
Michigan, and thence across Lake Superior the needle pointed
true north, while in the northeast corner of Maine the declina-
tion was 21 west and in the extreme northwest of the State of
Washington it was 24 east. The magnetic declination is some-
times called the magnetic variation, but there are several kinds of
magnetic variation and the term declination is to be preferred.

170. Isogonic Chart. It is of the utmost importance that
navigators should know the magnetic declination at whatever
point their vessel may be. For example, if a vessel be off the mouth
of the Columbia River and its captain, wishes to sail due north,
he must steer by compass 22 to the west of north. A knowledge
of the declination is also required by surveyors. Information of
this kind is often given graphically in so-called magnetic maps.
One of these, for the year 1905, is shown in Fig. 82 and is prepared
by joining by a continuous line all those points at which in that
year the declination was the same. The resulting curves are
called isogonic lines (lines of equal declination) and the map is
called an isogonic chart. The heavy lines are the agonic lines, or
lines of no declination; the lighter lines are those of westerly
declination; the dotted lines are those of easterly declination. It
will be noted that there is one agonic line completely encircling
the earth (shown as two in the Mercator's projection used in the
chart) and a second one embracing an elliptical area in eastern
Asia. This last is called the Siberian oval.

Figure 83 is the isogonic chart for the United States for the year
1905 taken from the report of the Superintendent of the Coast
Survey for 1906.

171. Magnetic Dip. In manufacturing needles for compasses
and surveying instruments, they are shaped and finished off while
the metal is soft, after which they are tempered glass hard and
then magnetized. They are carefully balanced before being
tempered for afterwards they are too hard to file and grinding
would injure their magnetization. Robert Norman, an instrument
maker of London, noticed in 1576 that no matter how carefully
he balanced his needles they were thrown out of balance after






being magnetized and invariably the north end appeared to be
the heavier so that he was compelled to restore the balance by
sticking a small piece of wax under the south end. Being angered
one day, or as he expressed it "being stroken into some choler,"
by ruining a needle upon which he had expended a good deal of
labor and whose balance he endeavored to restore by cutting off
a small piece from the north end, he began to reflect upon the
matter and finally made a needle which, before being magnetized,
balanced on horizontal trunnions. After magnetization, the
north end dipped down until the needle stood at an angle of 72
with the horizontal plane. The angle which the axis of such a
needle makes with the horizontal plane is called the magnetic dip
or magnetic inclination. The explanation of the magnetic dip is
as follows: The lines of force of the earth's magnetic field not
being circles and its poles being at some unknown depth, these
lines of force are not parallel to the surface but penetrate it, in
'Other words, they are inclined to the plane of the horizon. A
magnetic needle free to move in a vertical as well as in a horizontal
plane will place itself tangent to the lines of force and the angle
which these lines make with the horizontal plane is the magnetic
dip. As in the case of the declination, the dip is slowly changing.

The lack of balance in the needles of engineering instruments
is frequently corrected by wrapping a fine silver wire about the
south end of the needle.

172. Dipping Needle. A needle arranged to measure the angle
of dip is called a dipping needle. One of these is shown in Fig. 84.
The needle is ten or twelve inches long and is mounted upon a
steel knife-blade axis resting upon polished agate bearings. For
still more delicate observations an instrument is used in which the
needle is suspended at the center of a complete graduated circle
which may be rotated about a vertical axis and which, like a
surveyor's transit, is furnished with a slow motion screw by
which it may be accurately placed in the meridian. The angles
are read by verniers and microscopes and observations are mul-
tiplied so as to eliminate instrumental errors. For example, to
correct for the error due to the line joining the 90 marks at the
top and bottom of the graduated circle not being vertical, the
angle marked by the needle is read, the circle is then rotated 180
around its vertical axis, the angle is again read and the mean of
these observations is taken. To correct for the error due to the



axis of suspension of the needle not corresponding with the center
of the circle, both ends of the needle are read and the mean of these
readings is taken. To correct for
error due to the magnetic axis of
the needle not corresponding with
its geometric axis, the above ob-
servations are repeated with the
needle reversed from back to front
and these readings are combined
with the former ones. To correct
for error due to lack of mechanical
balance in the needle, observations
are made, the needle is then de-
magnetized and remagnetized in
the opposite direction, placed in
position, a second set of observa-
tions taken and the means of the
two sets combined. There are
observed other refinements not
necessary to mention here.

173. Isoclinic Chart. Figure 85

represents a section of the earth _

by a plane passing through its axis i?i g . 4.

and the north magnetic pole. The

arrows represent the position of the dipping needle at the corre-
sponding points. At the magnetic poles the dip is 90 or the

Fig. 85.

needle stands vertical and this was one of the observations by
means of which the north magnetic pole was located. Along




the magnetic equator, which in the western hemisphere lies south
of the geographical equator, the dip is zero or the needle lies
horizontal. Lines connecting those points on the earth's surface
where the dip is the same are called isoclinic lines. An examina-
tion of the isoclinic chart, Fig. 86, will show that these lines
run generally east and west but curve irregularly and are not

174. Magnetic Intensity. The strength of the earth's field,
or the magnetic intensity, can not easily be measured directly but
by the method outlined in Pars. 148, 149 and 150 we may deter-
mine its horizontal component, whence, since the total intensity
is equal to this horizontal component divided by the cosine of the
angle of dip, the total intensity is readily calculated.

Having determined the horizontal component at one point, it
may easily be determined at any other by applying the method
by oscillations as described in Par. 129. The same magnetic needle
is oscillated for the same period of time at the two places and the
number of oscillations counted; the horizontal components at the
two places are to each other as the square of the number of oscil-
lations executed in equal intervals of time.

The horizontal component is greatest along the magnetic equa-
tor but varies at different points along this line. It is a maximum
over a region embracing a part of India, the Malay Peninsula
and the Islands of Borneo and New Guinea, its strength being
.38, that is, a unit pole placed in the earth's field in this region
would be urged in a horizontal direction with a force of .38 dynes.
At the magnetic poles the horizontal component is zero and near
these points the total intensity is determined from the vertical
component instead of from the horizontal. The total intensity
increases from the equator towards the magnetic poles. It is
however not a maximum at these poles but in each hemisphere at
two points or magnetic foci. In the northern hemisphere one of
these points is just south of Hudson Bay, the other is in north
central Siberia. In the southern hemisphere both points are to
the south of Australia. The maximum value in the northern
hemisphere is about .65 and in the southern about .70. Just as
with the declination and the dip, the total intensity is found to be
slowly changing.

Lines connecting points of equal horizontal intensity or of equal
total intensity are called isodynamic lines, and isodynamic charts



are prepared in a similar manner to the isogonic and isoclinic

175. Magnetic Elements. The declination, the dip and the
magnetic intensity at any given point are termed the magnetic
elements of that point. As observations are multiplied, our knowl-
edge of these elements and of the laws of their variation corre-
spondingly increases. In the report of the Superintendent of the
Coast and Geodetic Survey for 1906, data is presented from ac-
curate observations at 3500 stations, or from every 30 miles
square of the U. S. territory. The following table, extracted from
this report, gives the declination, dip and horizontal intensity at
various localities as determined by observations made in the year
ending June 30, 1906.


Online LibraryWirt RobinsonThe elements of electricity → online text (page 11 of 46)