Copyright
Wirt Robinson.

The elements of electricity online

. (page 12 of 46)
Online LibraryWirt RobinsonThe elements of electricity → online text (page 12 of 46)
Font size
QR-code for this ebook


1905-1906.



Locality Declination

Albany, N. Y 11 08' W

Ann Arbor, Mich 2 01' W

Baltimore, Md 5 55' W

Bangor, Me 17 28' W

Columbia, S.C O c

Fargo, N. D ll c

Galveston, Tex T

Green River, Utah 15 C

Helena, Mont 19 C

Honolulu 1035'E

Joliet, 111 252'E

Key West, Fla 2 31' E

Los Angeles, Cal 15 C

Memphis, Tenn 5 C

Montreal, Can 14<

New York, N. Y 9<

Philadelphia, Pa T

Portland, Ore 22 C

San Francisco, Cal 17

Silver City, N. M 12

Sitka, Alaska 30 01' E

Washington, D. C 4 34' W



0'

30' E

28' E

:o 40' E

49' E



:o 14' E
:o 30' E
40' W
08' W
ro 45' W
44' E
00' E
46' E



Dip

73 50'
72 51'
70 42'
74 50'
65 35'
75 35'
58 37'
66 08'
72 08'
39 20'
72 13'
55 03'
59 31'
65 47'
75 38'
72 02'
71 04'
68 39'
62 43'
59 51'
74 42'
70 28'



Hor. Intensity

.16939
.18248
.19560
.15715
.23791
.15731
.28404
.23476
.18548
.29566
.18857
.29404
.26902
.24080
.15122
.18690
.19361
.21754
.24898
.27301
.15494
.20022



MAGNETISM.



137



176. Variation of the Magnetic Elements. The magnetic
elements at any locality are far from being constant. They pass
through cycles of variation with periods of years, through others
with the seasons, still others in each twenty-four hours and finally
others at irregular intervals. These variations may therefore be
classed as periodic and irregular, the first class embracing the
secular, the annual and the diurnal variations. Although all of
the elements vary, it is only to the variation in declination, and
furthermore only to the secular variation of this element, that
any practical importance attaches.

177. Secular Change in Declination and Dip. For over 300
years it has been noted that the decimation and dip were slowly
changing. In 1580 at London the declination was 11 17' east and
was decreasing so that in 1657 the needle pointed true north. The



1500



1600



1700



1800



1900



2000



variation west
20 10



variation east
10 20



Fig. 87.



movement continued in the same direction until in 1816 a maxi-
mum westerly declination of 24 30' was reached and retrogression
began. In 1900 the declination was 16 16' west. This movement
is shown graphically by the curve in Fig. 87. In about the year
1976, or some 320 years since the needle last pointed true north, it
should again point north, but since the curve shows that the



138 ELEMENTS OF ELECTRICITY.

westerly variation is greater than the easterly, the period of a
complete cycle will not be known until the needle moving westward
again points to 11 17' as in 1580.

The change in declination is accompanied by a change in dip,
although the angular range of the dip is much less than that of the
declination. At London the total variation in dip has been 7 33'
while that in declination has been 35 47'; however, the range in
dip is still increasing which is not true of the range in declination.
To the eye of the observer placed at the pivot of the needle in
London, the north pole of the needle would appear to have traced
in a clockwise direction since 1580 about two-thirds of a more or
less irregular and flattened oval. This fact, taken alone, would
seem to indicate that the north magnetic pole viewed from some
point outside of the earth is slowly rotating hi a counter-clockwise
direction around some undetermined point in the northern regions.

As we travel around a parallel of latitude we find, as has already
been shown, that the declination differs at different points and is
changing. We also find that both the direction of change and the
rate of change vary from place to place. Thus (Fig. 83) across
the northeast of Maine in 1905 the declination was 20 west and
was increasing 4' per year; along the agonic line through South
Carolina the declination was varying westerly 2' per year; along
a line through Alabama, Illinois and Wisconsin the declination
was stationary; along the Mississippi Valley it was increasing
easterly V per year; along the crest of the Rocky Mountains it
was increasing easterly 3' per year and on the coast of Oregon,
where the declination was 20 east, it was increasing easterly 4'
per year. These changes indicated that the isogonic lines were
slowly crowding in upon the agonic line and that the north mag-
netic pole was moving southward. As observations increase we
may be able in time to speak with more certainty of these move-
ments.

178. Diurnal Change in Declination. The magnetic elements
are subject to slight daily changes and these changes are more
satisfactorily studied by means of self recording instruments.
For instance, a needle suspended by a delicate silk fibre carries a
small concave mirror upon which falls a beam of light from an
electric bulb. This mirror reflects a brilliant spot upon a roll of
photographic paper which is unwound at a known rate by clock-
work. As long as the needle is motionless, the trace of the spot



MAGNETISM.



139



upon the sensitized paper is a straight line but any movement of
the needle produces a curve. Fig. 88 represents such a record
made near London in 1900. At 8 A. M. the declination was least
but increased steadily until about 2 P. M. when it reached a
maximum. It then decreased until 8 P. M. when it was nearly
stationary for about an hour and then began to decrease again
and continued until 8 A. M. A similar record would be made at
all points, no matter whether the local declination be east or west,
but the direction of movement in the southern hemisphere is the
reverse of that in the northern. Along the equator the daily



12



P.M.



A.M



PH:



P.M.



Fig. 88.



range of the needle does not exceed 4' while in northern Europe
it reaches 15'.

The dip, registered in a similar manner, is found to be about 5'
greater in the morning than in the afternoon.

179. Annual Change in Declination. If the average declination
for each month be obtained from the self -registering instruments
and these monthly averages be compared among themselves, it
will be seen that in the northern hemisphere the needle moves to
the west from May to September and to the east from September
to May. In the southern hemisphere these movements are re-
versed and in either case they are but slight.

180. Magnetic Storms. It has long been known that in addi-
tion to the periodic variations described in the preceding para-



140



ELEMENTS OF ELECTRICITY.



graphs, magnetic needles are not infrequently subject to other
variations occurring at irregular intervals. If a needle be observed
at such a time it will be seen to waver or tremble and to fluctuate
through an angle varying from a few minutes to one degree and in
extreme cases even to two to three degrees. The variation is only
momentary but may be often repeated. Such disturbances are
called magnetic storms. They occur simultaneously at the most
distant points and involve all the magnetic elements. Their
effects are best studied by means of the curves traced as described
in Par. 178. The record instead of being the sinuous curve as in
Fig. 88 is jagged and irregular. These storms occur more fre-
quently at night than during the day and are also more frequent
in summer than in winter. They are especially marked during
auroral displays and it was for a time thought that the two phe-
nomena were related as effect and cause, but it is now held that
they have a common cause.



1810 1820 1830 1840 1850 18.60 1870 J8fcO

Fig. 89.

In 1852 it was observed that the periods of maximum frequency
of magnetic storms coincides with the maximum occurrence of
sun spots, both taking place every eleventh year. This coin-
cidence is shown graphically in Fig. 89 in which the full line shows
the relative number of sun spots for each year and the broken line
the number of magnetic storms. The agreement is too close to be
accidental.

181. Theories of the Earth's Magnetism. There is no accepted
theory of the earth's magnetism but since, as we have seen above,
its manifestations are periodic in character, these periods corre-
sponding to the diurnal and annual time periods of the earth and



MAGNETISM. 141

to the eleven year period of the sun spots, the indications are that
its source is the sun. The significance of the declination period
has not yet been grasped, in fact, as was pointed out (Par. 177),
we can not be sure for a number of years to come what is the
exact length of this period. Could it be shown that electric cur-
rents flowed around the globe from east to west, this, as will be
seen in electro-magnetics, would account for the magnetic phe-
nomena and this explanation was advanced and elaborated by
Ampere. So-called earth currents are known to exist but their
direction is along the meridian instead of across it. It is known
that electricity is produced both by heat and by evaporation, also
that the magnetic properties of bodies are effected by heat, and it
is conceivable that the sun as in its apparent motion it sweeps
along overhead at the equator at the rate of 1000 miles per hour
may produce successive masses of charged vapor which might
have an effect similar to a current, and also that the warming of
successive portions of the earth's crust may alter its magnetic
properties sufficiently to account for the diurnal and seasonal
variations. An additional fact which points to this hypothesis is
that the isothermal lines, or lines of equal average temperature of
the earth's surface, correspond closely in direction with the
isoclinal lines.

Faraday, in investigating paramagnetic and diamagnetic bodies,
discovered that oxygen is magnetic and that its magnetism in-
creases as it grows colder. He therefore suggested that the oxygen
of the atmosphere is naturally magnetic and that the variations
produced in its magnetism by the daily and seasonal variations
in temperature would afford a satisfactory explanation of the
periodic variations of the needle.

Other theories have been advanced but they can not be regarded
as much more than speculations.

182. The Mariner's Compass. In the surveyor's compass, a
long, slender needle is pivoted free to rotate within a horizontal
circle which is so graduated that the north end of the needle points
to the angle which the line of sight of the telescope makes with the
magnetic meridian. Since the graduated circle and the telescope
rotate together about the vertical axis of the instrument, the
needle remaining motionless, the west half of the circle must be
marked east and the east half must be marked west.



142 ELEMENTS OF ELECTRICITY. }

The mariner's compass (Fig. 90) is differently arranged, the
graduated scale being fastened to the needle and rotating with it
and hence the interchange of east and west not being necessary.
The pointer which indicates the direction in which the vessel is




Fig. 90.

sailing is a vertical mark on the inside of the box in which the
compass turns. In the compass perfected by Lord Kelvin there
turns upon an iridium needle point a central jewelled cup to which
is attached by tightly drawn silk threads a thin aluminum ring,
six or eight inches in diameter, the whole resembling a wheel of
which the cup is the hub and the threads the spokes. Upon the
rim is fastened the paper scale divided into the customary 32
"points of the compass," and also with an outer graduation in
degrees. The needle proper consists of eight separate needles,
slender bars about three inches long, arranged like the rungs of a
ladder and fastened to the under side of the silk spokes, being
symmetrically placed with respect to the jewelled cup. The com-
pass is contained in a glass-covered, cylindrical copper box,
weighted at the bottom and supported on gimbals. To the box
itself there are attached two trunnions which rest upon a copper
ring concentric with the box. This ring in turn carries two trun-
nions which are in the same horizontal plane as the first pair and
at right angles to them, and these in their turn rest upon a second
and outer concentric ring. By this arrangement the compass is
kept horizontal no matter how much the ship may roll. In order
to slow down the oscillations of the needle, the box is often filled
with some thick non-freezing liquid, such as glycerine, and by
making a portion of the rim of the compass card hollow, the liquid



MAGNETISM. 143

will buoy up the card and relieve the pivot of a portion of the
weight upon it.

The compass and its box are placed upon a pedestal, called the
binnacle, which carries the necessary lamps for reading the com-
pass at night and also supports the magnets and masses of soft
iron used in making correction for local disturbances of the needle.

183. Adjustment of Mariner's Compass. In the construction
of vessels the use of iron and steel has largely displaced wood.
During the building of a vessel it rests for a relatively long period
of time at a constant angle with the earth's field and the continual
hammering and vibration to which it is subjected converts it as a
whole into a magnet. In addition to this, such vertical columns of
steel as the cut water and the stern post become, as explained in
Par. 156, magnets whose south poles, for vessels in the northern
hemisphere, are at the upper ends and therefore about on a level
with the deck upon which the compass stands. When such a
vessel is launched the" magnetism of the hull may entirely vitiate
the indications of the compass. However, it has been found that
by means of permanent magnets and of masses of iron, properly
placed, compensation may be made for these disturbing influences.
For example, reflection will show that a magnetic cut water and
stern post produce no variation in the compass when the vessel
is sailing in the magnetic meridian, either north or south, but if it
be sailing in any other direction in the semicircles to the east or
west, the compass will be affected. Since the error produced is in
one semicircle always to the east and in the other always to the
west, the disturbance is called the semicircular variation. It may
be corrected by a vertical rod of iron or a sphere of soft iron placed
on the opposite side of the binnacle from the vertical magnetic
body whose influence is the stronger. Similarly, the magnetism
of the hull may be divided into two components, one lengthwise of
the ship, the other crosswise, and these can be separately counter-
balanced by compensating magnets placed usually in the pedestal
of the binnacle. In making these adjustments, the newly launched
vessel is anchored in some known position with reference to the
magnetic meridian and the needle is brought to its correct reading.
The vessel is then swung through an angle of 90 and adjustments
again made, and so on around the circle, the process being called
swinging ship. Magnetic masses in the cargo may cause disturb-
ances of the needle and the magnetism of the hull grows less with



144 ELEMENTS OF ELECTRICITY.

age and varies with the latitude, the vertical component becoming
entirely reversed when the magnetic equator is crossed, therefore
the navigator checks the indications of his needle by frequent
astronomical observations and makes the necessary adjustments
when the error becomes excessive.

184. Magnetism to be Reverted to Later. The subject of
magnetism is usually treated more extensively than in the pre-
ceding chapters. Thus, a theory of magnetic potential may be
developed similarly to that of electric potential. It is thought,
however, that enough of the principles have been given to enable
the student to follow without difficulty the explanations in the
following sections. Moreover, in view of the fact that electro-
magnets, or magnets produced temporarily by means of the
electric current, are for most purposes far more suitable and more
largely used than permanent magnets, and that the phenomena
and properties of the magnetic circuit are most markedly exhibited
and can be most clearly explained by reference to these electro-
magnets, it is logical that we should first take up the subject of
electric currents. Further consideration of magnetism is there-
fore postponed for the present. (See Chapters 31 and 32.)



VOLTAIC ELECTRICITY. 145



PART III.
VOLTAIC ELECTRICITY.



CHAPTER 18.

DISCOVERIES OF GALVANI AND VOLTA.

185. Galvani's Discovery. The discovery of current electricity,
or rather of methods of producing it by chemical means, is as-
cribed to two Italians, Galvani and Volta, the former Professor of
Anatomy at Bologna, the latter Professor of Natural Philosophy
at Pavia.

Tradition has it that about 1786 the wife of Galvani being indis-
posed, her physician prescribed for her a broth of frogs' legs.
Some had been procured and skinned preparatory to cooking and
lay upon a table near an electrical machine. Galvani's assistant
happening to draw a spark from the machine, Madame Galvani
noticed that at the same instant the severed legs twitched convul-
sively and that this was repeated with every spark. She called the
attention of her husband to this phenomenon which he imme-
diately proceeded to investigate. We now know that these twitch-
ings were produced by the escape of the charge induced in the
legs, which charge was released whenever the machine sparked,
but Galvani, who was an anatomist and not an electrician, thought
that he was on the verge of discovering the vital principle and
continued his researches with this idea in mind. Having one day
prepared several pairs of legs for experiment and wishing to place
them to one side until they were needed, he hooked a copper wire
through the remnant of the back bone and hung the legs to the
iron railing of the balcony in front of his window. A light wind
was blowing and to his astonishment he saw that whenever the
dangling legs came in contact with the railing they were thrown
into convulsive movement. Further experiment showed him that
in order to produce these movements it was necessary to have two



146



ELEMENTS OF ELECTRICITY.



dissimilar metals in contact and that the greatest effect was pro-
duced when the free end of one touched a nerve at the same time
that the free end of the other touched a muscle. He attributed
these effects to a so-called "animal electricity" whose seat lay at
the junction of the nerve and muscle, where, by some unknown
vital principle, the nerve became charged positively and the
muscle negatively, and, like the Leyden jar, were discharged when
connected by the metals. He did not explain why two metals
were required.

186. Volta's Investigations. Volta was not long in hearing of
these experiments and, favored by his greater familiarity with
what was then known of electricity, pursued a line of investigation
which soon satisfied him that the true seat of development of the
electricity was not at the junction of the muscle and the nerve
but at the point of contact of the two metals. He found that if
two dissimilar metals are brought together, one becomes positively
charged, the other negatively, that is, they become of different
potentials. This electrification by contact may be shown as fol-
lows. In Fig. 91, A is a light flat needle suspended symmetrically
above the gap between the semicircular plates of zinc and copper
and free to turn about the vertical axis X. If a positive charge be
given to A and if then the copper and zinc plates be brought into
contact at B, either by touching them together directly or by




Fig. 91.



laying a piece of wire across the gap, the needle will swing away
from the zinc and place itself above the copper, thus apparently
showing the zinc to be positively charged or at a higher potential
than the copper. Had the needle been charged negatively, it
would have swung away from the copper and placed itself above
the zinc.



VOLTAIC ELECTRICITY. 147

187. Volta's Contact Series. Further investigation by Volta
showed that for a given pair of metals at a constant temperature,
this contact difference of potential is constant and is independent
of the size of the pieces, of the amount of surface in contact and
of the length of time that they remain in contact. For different
pairs of metals, however, it varies with the particular ones used,
and he was able to draw up a list of these, similar to the list of
substances given in Par. 23, so arranged that any one becomes
positively electrified when touched to those below it in the series
but negatively electrified when touched to those above it. Volta's
list comprised seven of the commoner metals. Such a list now
would be headed by the alkaline metals, unknown in Volta's
time, and would be ended by the non-metal carbon. His observa-
tions were merely qualitative but subsequent observers have
accurately measured these differences of potential. In the follow-
ing list the numbers between the names indicate the difference in
potential in volts set up between the corresponding pairs of metals
when placed in contact:

Zinc

.210 volt
Lead

.069 volt
Tin

.313 volt
Iron

.146 volt
Copper

.238 volt
Platinum

.113 volt
Carbon

The difference of potential between any two metals in the series
is the sum of the intervening numbers. Thus, with a zinc and
copper pair, the difference would be .738 volts and between zinc
and carbon it is 1.089 volts.

Regarding as negative the difference of potential between any
pair taken in reverse order from that given in the above list, it
follows that the difference in potential between the first and last
metals of any number in series depends only upon these two and



148 ELEMENTS OF ELECTRICITY.

is independent of the intervening metals or of the order in which
they are arranged. Also, no matter how the intervening metals
may be arranged, there is no difference of potential between the
ends of a series beginning and ending with the same metal.

The foregoing list might be extended to include other substances
than the metals. For example (and this fact is extremely impor-
tant), a difference of potential is produced between a metal and a
liquid when brought into contact and if the liquid attacks the
metal chemically, an electro-motive force will act from the metal
towards the liquid. Finally, a difference of potential is produced
when two liquids come into contact and even between solutions
of the same substance when these solutions are of different degrees
of concentration.

188. Volta's Contact Theory. While there is no uncertainty as
to the facts as set forth above, there has been much controversy
as to the interpretation to be put upon them. According to Volta,
when two dissimilar metals are brought together, the surface of
contact becomes a seat of electro-motive force which drives posi-
tive electricity in one direction from the junction and negative
electricity in the opposite, and this separation continues until the
force of attraction between the dissimilar charges balances the



A

f~


- COPPER -


-* ZINC +1 - ^


i
t

V




i
i

/



Fig. 92.

force which drives them apart. Thus in the compound bar of
copper and zinc, Fig. 92, the zinc end becomes positively charged,
the copper end negatively, or, the zinc end is at a higher potential
than the copper.

In general, when bodies at different potentials are connected by
a conductor, there is a flow of electricity from the one of higher
potential to the one of lower, and, unless constantly re-established,
the difference of potential disappears. It would therefore seem
that in this case if B be connected to A by a wire, a flow of elec-
tricity would take place from B to A, but it can be shown that
where the difference of potential is produced by contact as above
and the metals are at the same temperature, it is not possible to
get such a flow. If, for example, the connecting wire be of copper or



VOLTAIC ELECTRICITY. 149 X

of zinc, the effect is the same as if the bar in Fig. 92 had been bent
around into a circle until the ends A and B touched, and when
these ends touch, a contact electro-motive force is set up equal
but opposite to the one already existing and hence just counter-
balancing it. If the wire be of some third metal, it follows from
Par. 187 that to whichever end of the bar it be connected, the
electrical effect is to convert the bar into a compound one con-
sisting of the metal of the remaining end and of that of the wire,,
and, as shown above, no current would be produced upon com-
pleting the circuit.

Independent theoretical considerations lead to the same con-
clusion, for if a current flowed through the wire joining B and A
in Fig. 92, by suitable arrangements, as we shall see later, this



Online LibraryWirt RobinsonThe elements of electricity → online text (page 12 of 46)