Edward Salisbury Dana.

A Century of science in America, with special reference to the American journal of science, 1818-1918 online

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matic investigation of the effect of electricity on the mag-
netic needle. His researches were without result until
during the course of a series of lectures on "Electricity,
Galvanism, and Magnetism M delivered during the winter
of 1819-20 it occurred to him to investigate the action of
an electric current on a magnetic needle. At first he
placed the wire bearing the current at right angles to the
needle, with, of course, no result; then it occurred to
him to place it parallel. A deflection was observed, for
to his surprise the needle insisted on turning until per-
pendicular to the wire.

Oersted's discovery that an electric current exerts a
couple on a magnetic needle was followed a few months
later by Ampere's demonstration before the French
Academy that two currents flowing in the same direction
attract each other, while two in opposite directions repel.
The story goes that a critic attempted to belittle this dis-
covery by remarking that as it was known that two cur-
rents act on one and the same magnet, it was obvious
that they would act upon each other. Whereupon Arago
arose to defend his friend. Drawing two keys out of
his pocket he said, "Each of these keys attracts a mag-
net; do you believe that they therefore attract each
other f"


A few years later Ampere showed how to express
quantitatively the force between current elements, and
indeed developed to a considerable degree the equiva-
lence between a closed circuit carrying a current and a
magnetic shell. So convincing was his analysis and so
thorough his discussion of the subject, that Maxwell said
of this memoir half a century later, "The whole, theory
and experiment, seems as if it had leaped, full grown and
full armed, from the brain of the i Newton of electricity.'
It is perfect in form and unassailable in accuracy; and
it is summed up in a formula from which all the phe-
nomena may be deduced, and which must always remain
the cardinal formula of electrodynamics."

Shortly afterwards the dependence of a current on the
conductivity of the wire used and the grouping of cells
employed, was made clear by the work of Ohm. Many
of his results were obtained independently by Joseph
Henry (19, 400, 1831) of the Albany Academy, who
described in 1831 a powerful electromagnet in which a
great many coils of wire insulated with silk were wound
around an iron core and connected in parallel with a sin-
gle cell. He remarks in this paper that with long wires,
as in the telegraph, many cells arranged in series should
be used, whereas for several short wires connected in
parallel a single cell with large plates is more efficient.

Current Induction. Impressed by the fact that elec-
tric charges have the power of inducing other charges
on neighboring conductors without coming into contact
with them, Faraday was engaged in investigating the
possibility of an analogous phenomenon in the case of
electric currents. His idea at first seems to have been
that a current should induce another current in any
closed conducting circuit which happens to be in its
vicinity. Experiment readily showed the falsity of this
conception, but a brief deflection of the galvanometer in
the secondary circuit was noticed at the instant of mak-
ing and breaking the current in the primary. Further
experiments showed that thrusting a permanent steel
magnet into a coil connected to a galvanometer caused
the needle to deflect. In fact Faraday's report to the
Royal Society on November 24th, 1831, contains a com-


plete account of all experimental methods available for
inducing a current in a closed circuit.

While Faraday is entitled to credit for the discovery of
current induction by virtue of the priority of his publica-
tion, it must not pass unnoticed that Henry obtained
many of the same experimental results independently
and some even earlier. Henry was at this time instruc-
tor in mathematics at the Albany Academy, and seven
hours of teaching a day made it well-nigh impossible to
carry on original research except during the vacation
month of August. As early as the summer of 1830 he
had wound 30 feet of copper wire around the armature
of a horseshoe electromagnet and connected it to a gal-
vanometer. When the magnet was excited, a momen-
tary deflection was observed. "I was, however, much
surprised," he says, "to see the needle suddenly
deflected from a state of rest to about 20 to the east, or
in a contrary direction, when the battery was withdrawn
from the acid, and again deflected to the west when
it was re-immersed." In addition a deflection was
obtained by detaching the armature from the magnet,
or by bringing it again into contact. Had the results of
these experiments been published promptly, America
would have been entitled to credit for the most import-
ant discovery of the greatest of England's many great
experimenters. But Henry desired first to repeat his
experiments on a larger scale, and while new magnets
were being constructed, the news of Faraday's discovery
arrived. This occasioned hasty publication of the work
already done in an appendix to volume 22, 1832, of the

At almost the same time Henry made another import-
ant discovery and this time he was anticipated by no
other investigator in making public his results. In the
paper already referred to he describes the phenomenon
known to-day as self-induction. "When a small battery
is moderately excited by diluted acid and its poles, which
must be terminated by cups of mercury, are connected by
a copper wire not more than a foot in length, no spark
is perceived when the connection is either formed or
broken ; but if a wire thirty or forty feet long be used,


instead of the short wire, though no spark will be per-
ceptible when the connection is made, yet when it is
broken by drawing one end of the wire from its cup of
mercury a vivid spark is produced. . . . The effect
appears somewhat increased by coiling the wire into a
helix ; it seems to depend in some measure on the length
and thickness of the wire ; I can account for these phe-
nomena only by supposing the long wire to become
charged with electricity which by its reaction on itself
projects a spark when the connection is broken."

Soon after, Henry went to Princeton and there con-
tinued his experiments in electromagnetism. No diffi-
culty was experienced in inducing currents of the third,
fourth and fifth orders by using the first secondary as
primary for yet another secondary circuit, and so on
(38, 209, 1840). The directions of these currents of
higher orders when the primary is made or broken
proved puzzling at first, but were satisfactorily explained
a year later (41, 117, 1841). In addition induced cur-
rents were obtained from a Leyden jar discharge. Fara-
day failed to find any screening effect of a conducting
cylinder placed around the primary and inside the
secondary. Henry examined the matter, and found that
the screening effect exists only when the induced current
is due to a make or break of the primary circuit, and not
when it is caused by motion of the primary.

Henry 's work was mainly descriptive ; it remained for
Faraday to develop a theory to account for the phenomena
discovered and to prepare the way for quantitative for-
mulation of the laws of current induction. This he did in
his representation of a magnetic field by means of lines
of force ; a conception which he found afterwards to be
equally valuable when applied to electrostatic problems.
Every magnet and every current gives rise to these
closed curves; in the case of a magnet they thread it
from south pole to north, while a straight wire bearing
a current is surrounded by concentric rings. The con-
nection between lines of force and the induction of cur-
rents is contained in the rule that a current is induced in
a closed circuit only when a change takes place in the
number of lines of force passing through it. Further-
more the dependence of the current strength on the


conductivity of the wire employed has led to recognition
of the fact that it is the electromotive force and not the
current itself which is conditioned by the change in mag-
netic flux.

Great interest was attached to the utilization of the
newly discovered forces of electromagnetism. In 1831
Henry (20, 340, 1831) described a reciprocating engine
depending on magnetic attraction and repulsion, and C.
G. Page (33, 118, 1838; 49, 131, 1845) devised many
others. The latter >s most important work, however, was
the invention of the Ruhmkorff coil. In 1836 (31, 137,
1837) he found the strongest shocks to be obtained from a
secondary coil of many windings forming a continuation
of a primary of half the number of turns. His perfec-
tion of the self-acting circuit breaker (35, 252, 1839)
widened the usefulness of the induction coil, and his sub-
stitution of a bundle of iron wires for a solid iron core
(34, 163, 1838) greatly increased its efficiency.

Conservation of Energy. Perhaps the most important
advance of the nineteenth century has been the estab-
lishment of the principle of conservation of energy.
Despite the fact that the "principe de la conservation des
force vives" had been recognized by the French mathe-
maticians of the early part of the century, the application
of this principle even to purely mechanical problems was
contested by some scientists. Through the early num-
bers of the Journal runs a lively controversy as to
whether there is not a loss of power involved in impart-
ing momentum to the reciprocating parts of a steam
engine only to check the motion later on in the stroke.
Finally Isaac Doolittle (14, 60, 1828), of the Bennington
Iron Works, ends the discussion by the pertinent remark :
"If there be, as is contended by one of your correspond-
ents, a loss of more than one third of the power, in trans-
forming an alternating rectilinear movement into a
continuous circular one by means of a crank, I should
like to be informed what would be the effect if the propo-
sition were reversed, as in the case of the common
saw mill, and in many other instances in practical
mechanics. ' '

A realization of the equivalence of heat and mechani-
cal work did not come until the middle of the century, in


spite of the conclusive experiments of the American
Count Rumford and the English Davy before the year
1800. So firmly enthroned was the caloric theory,
according to which heat is an indestructible fluid, that
evidence against it was given scant consideration. In
fact the success of the analytical method introduced by
Fourier in 1822 for the solution of problems in conduc-
tion of heat only added to the difficulties of the adherents
of the kinetic theory. But recognition of heat as a form
of energy was on the way, and when it came it made its
appearance almost simultaneously in half a dozen differ-
ent places. Perhaps Robert Mayer of Heilbronn was
the first to state explicitly the new principle. His paper
"On the Forces of Inorganic Nature" was refused
publication in Poggendorff 's Annalen, but fared better at
the hands of another editor. During the next few years
Joule determined the mechanical equivalent of heat
experimentally by a number of different methods, some
of which had already been devised by Carnot. Of those
he used, the most familiar consists in churning up a
measured mass of water by means of paddles actuated by
falling weights and calculating the heat developed from
the rise in temperature. However, the work of the
young Manchester brewer received little attention from
the members of the British Association before whom it
was reported until Kelvin showed them its significance
and attracted their interest to it. Meanwhile Helmholtz
had completed a very thorough disquisition on the con-
servation of energy not only in dynamics and heat but in
other departments of physics as well. His paper on
"Die Erhaltung der Kraft" was frowned upon by the
members of the Physical Society of Berlin before whom
he read it, and received the same treatment as Mayer's
from the editor of PoggendorfPs Annalen. Helmholtz 's
"Kraft," like the "vis viva" of other writers, is the
quantity which Young had already christened energy.
Not many years elapsed, however, until the convictions of
Mayer, Joule, Kelvin and Helmholtz became the most
clearly recognized of all physical principles. As early
as 1850 Jeremiah Day (10, 174, 1850), late president^ of
Yale College, admitted the improbability of constructing


a machine capable of perpetual motion, even though the
"imponderable agents " of electricity, galvanism and
magnetism be utilized.

Thermodynamics. The importance of the principle of
conservation of energy lies in the fact that it unites under
one rule such diverse phenomena as gravitation, electro-
magnetism, heat and chemical action. Another principle
as universal in its scope, although depending upon the
coarseness of human observations for its validity rather
than upon the immutable laws of nature, was fore-
shadowed even before the first law of thermodynamics,
or principle of conservation of energy, was clearly
recognized. This second law was the consequence of
efforts to improve the efficiency of heat engines. In 1824
Carnot introduced the conception of cyclic operations
into the theory of such engines. Assuming the impos-
sibility of perpetual motion, he showed that no engine can
have an efficiency greater than that of a reversible
engine. Finally Clausius expressed concisely the princi-
ple toward which Carnot 's work had been leading, when
he asserted that "it is impossible for a self-acting
machine, unaided by any external agency, to convey heat
from one body to another at a higher temperature. "
Kelvin's formulation of the same law states that "it is
impossible, by means of inanimate material agency, to
derive mechanical effect from any portion of matter by
cooling it below the temperature of the coldest of the
surrounding objects. ' '

The consequences of the second law were rapidly
developed by Kelvin, Clausius, Rankine, Barnard (16,
218, 1853, et seq.) and others. Kelvin introduced the
thermodynamic scale of temperature, which he showed
to be independent of such properties of matter as con-
dition the size of the degree indicated by the mercury
thermometer. This scale, which is equivalent to that of
the ideal gas thermometer, was used subsequently by
Rowland in his exhaustive determination of the mechan-
ical equivalent of heat by an improved form of Joule's
method. He found different values for different ranges
in temperature, showing that the specific heat of water
is by no means constant. Since then electrical methods


of measuring this important quantity have been used to
confirm the results of purely mechanical determinations.

The definition of a new quantity, entropy, was found
necessary for a mathematical formulation of the second
law of thermodynamics. This quantity, which acts as a
measure of the unavailability of heat energy, was given
a new significance when Boltzmann showed its connec-
tion with the probability of the thermodynamic state of
the substance under consideration. If two bodies have
widely different temperatures, a large amount of the
heat energy of the system is available for conversion
into mechanical work. From the macroscopic point of
view this is expressed by saying that the entropy is small,
or if the motions of the individual molecules are taken
into account, the probability of the state is low. The
interpretation of entropy as the logarithm of the thermo-
dynamic probability has thrown much light on the
meaning of this rather abstruse quantity. Gibbs's
' ' Elementary Principles in Statistical Mechanics ' ' treats
in detail the fundamental assumptions involved in
this point of view, its limitations and its consequences.
In his "Equilibrium of Heterogeneous Substances m
he had already extended the principle of thermal equi-
librium to include substances which are no longer homo-
geneous. The value of the chemical potential he intro-
duced determines whether one phase is to gain at the
expense of another or lose to it. It is unfortunate that
the analytical rigor and austerity of his reasoning com-
bined with lack of mathematical training on the part of
the average chemist, delayed true appreciation of his
work and full utilization of the new field which he
opened up.

Liquefaction of Gases. 'Meanwhile the problem of
liquefying gases was attracting much attention on the
part of experimental physicists. Faraday had succeeded
in making liquid a number of substances which had
hitherto been known only in the gaseous state. His
method consists in evolving the gas from chemicals
placed in one end of a bent tube, the other end of which
is immersed in a freezing mixture. The high pressure
caused by the production of the gas combined with the
low temperature is sufficient to bring about liquefaction


in many cases. Failure with other more permanent
gases was unexplained until the researches of Andrews
in 1863 showed that no amount of pressure will produce
liquefaction unless the temperature is below a certain
critical value. The method of reducing the temperature
in use to-day depends on a fact discovered by Kelvin and
Joule in connection with the free expansion of a gas.
These investigators allowed the gas to escape through a
porous plug from a chamber in which the pressure was
relatively high. With the single exception of hydrogen,
the effect of the sudden expansion is to cool the gas, and
even with it cooling is found to take place after the tem-
perature has been made sufficiently low. By this method
all known gases have been liquefied. Helium, with a
boiling point of 269 C., or only 4C. above the absolute
zero, was the last to be made a liquid, finally yielding to
the efforts of Kammerlingh Onnes in 1907. This inves-
tigator 2 finds that at temperatures near the absolute zero
the electrical conductivity of certain substances undergoes
a profound modification. For example, a coil of lead
shows a superconductivity so great that a current once
started in it persists for days after the electromotive
force has ceased to act.

Electrodynamics. Faraday's representation of elec-
tric and magnetic fields by lines of force had been of
great value in predicting the results of experiments in
electromagnetism. But a more mathematical formula-
tion of the laws governing these phenomena was needed
in order to make possible quantitative development of
the theory. This was supplied by Maxwell in his
epoch-making treatise on "Electricity and Mag-
netism." Starting with electrostatics and magnetism,
he gives a complete account of the mathematical
methods which had been devised for the solution
of problems in these branches of the subject, and
then turning to Ampere's work he shows how the
Lagrangian equations of motion lead to Faraday's law
if the single assumption is made that the magnetic
energy of the field is kinetic. In the treatment of open
circuits Maxwell's intuition led to a great advance, the
introduction of the displacement current. Consider a
charged condenser, the plates of which are suddenly con-


nected by a wire. A current will flow through the wire
from the positively charged plate to the negative, but in
the gap between the two plates the conduction current
is missing. So convinced was Maxwell that currents
must always flow in closed circuits, that he postulated an
electrical displacement in the medium between the plates
of a charged condenser, which disappears when the con-
denser is short-circuited. Thus even in the so-called
open circuit the current flows along a closed path.

Maxwell's theory of the electromagnetic field is based
essentially on Faraday's representation by lines of force
of the strains and stresses of a universal medium. So it
is not surprising that he was led to a consideration of
the propagation of waves through this medium. The
introduction of the displacement current made the form
of the electrodynamic equations such as to yield a typical
wave equation for space free from electrical charges and
currents. Moreover, the disturbance was found to be
transverse, and its velocity turned out to be identical
with that of light. The conclusion was irresistible.
That light could consist of anything but electromagnetic
waves of extremely short length was inconceivable. In
fact so certain was Maxwell of this deduction from
theory that he felt it altogether unnecessary to resort to
the test of experiment. For the electromagnetic theory
explained so many of the details which had been revealed
by experiments in light, that no doubt of its validity
could be entertained. Even dispersion received ready
elucidation on the assumption that the dispersing
medium is made up of vibrators having a natural period
comparable with that of the light passing through it.

Maxwell's book was published in 1873. Fifteen years
later, Hertz, 3 at the instigation of Helmholtz, succeeded
in detecting experimentally the electromagnetic waves
predicted by Maxwell's theory. His oscillator consisted
of two sheets of metal in the same plane, to each of which
was attached a short wire terminating in a knob. The
knobs were placed within a short distance of each other,
and connected to the terminals of an induction coil. By
reflection standing waves were formed, and the positions
of nodes and loops determined by a detector composed of
a movable loop of wire containing an air gap. Thus the


wave length was measured. Hertz calculated the fre-
quency of his radiator from its dimensions, and then
computed the velocity of the disturbance. In spite of an
error in his calculations, later pointed out by Poincare,
he obtained very nearly the velocity of light for waves
traveling through air, but a velocity considerably smaller
for those propagated along wires. Subsequent work by
Lecher, Sarasin and de la Rive, and Trowbridge and
Duane (49, 297, 1895; 50, 104, 1895) cleared up this dis-
crepancy, and showed the velocity to be in both cases
identical with that of light. The last-named investiga-
tors increased the size of the oscillator until it was possi-
ble to measure the frequency by photographing the spark
in the secondary with a rotating mirror. The positions
of nodes and loops were obtained by means of a bolom-
eter after the secondary had been tuned to resonance
with the vibrator. The velocity thus found for electro-
magnetic waves along wires is within one-tenth of one
percent of the accepted value of the velocity of light.
Hertz's later experiments showed that waves in air suf-
fer refraction and diffraction, and he succeeded in
polarizing the radiation by passing it through a grating
constructed of parallel metallic wires.

In order to satisfy the law of action and reaction, it
is found necessary to attribute a quasi-momentum to
electromagnetic waves. When a train of such waves is
absorbed, their momentum is transferred to the absorb-
ing body, while if they are reflected an impulse twice as
great is imparted. This consequence of theory, foreseen
by Maxwell and developed in detail by Poynting, Abra-
ham and Larmor, has been verified by the experiments of
Lebedew, and Nichols and Hull. 4 The latter used a deli-
cate torsion balance from which was suspended a couple
of silvered glass vanes. In order to eliminate the effect
of impulses imparted by the molecules of the residual
gas, such as Crookes had observed in his radiometer,
readings were made at many different pressures and the
ballistic rather than the static deflection recorded.
After the pressure produced by light from a carbon arc
had been measured, the intensity of the radiation was
determined with a bolometer. Preliminary experiments
indicated the existence of a pressure of the order


expected, and later more careful measurements showed
good quantitative agreement with theory. This pressure
had already found an important application in Lebedew's
explanation of the solar repulsion of comet's tails.
These tails are made up of enormous swarms of very
minute particles, and as the comet swings around the
sun they suffer a repulsion due to the pressure of the
intense solar radiation which counteracts the sun's gravi-
tational attraction. Hence the tail, instead of following

Online LibraryEdward Salisbury DanaA Century of science in America, with special reference to the American journal of science, 1818-1918 → online text (page 31 of 41)