of a tuning-fork and then let go, the prongs will return to
their normal position after performing a number of vibrations of
gradually diminishing amplitude. The state of rest is reached when
the energy of bending, produced by the work of our fingers, has
been expended, partly in overcoming the internal friction in the
tuning-fork, partly in overcoming the reactions of the surrounding
medium, the air; this last effect results in sound-waves which are
radiated off into space. The stiffness and the mass of the prongs
of the fork determine the period of vibration, that is, the pitch
of the fork.
I confess that in the course of my life since my Berlin days I
afforded considerable amusement to my friends whenever I tried
to explain to them the Hertzian experiments by appealing to
what I considered a well-known action of the tuning-fork. Some
of them objected on the ground that this action is just as
difficult to understand as the action of the Hertzian oscillator.
I met this objection by describing to them the action of the
reed in Serbian bagpipes which I watched when I was a boy, and
understood sufficiently well to recognize later in the action of
the tuning-fork a performance similar to that of the reed in the
Serbian bagpipes. I understood the tuning-fork because I understood
the reed. An educated American, I claimed, should find no
difficulty in understanding the action of a simple mechanism which
an uneducated Serbian peasant boy understood.
The Hertzian electrical oscillator, described above, acts like the
tuning-fork. The process of pulling apart the two charges, the
positive from the negative, and of forcing them to the surface of
the spheres by the action of the electrical machine, is a parallel
to the process of deflecting by the pressure of our fingers the
prongs of the tuning-fork from their normal position. In one case
the tuning-fork by its elastic stiffness reacts against the bending
of the prongs. In the electrical case the electrical lines of force
in the space surrounding the oscillator react against the action
of the machine which crowds them into this space by stretching and
compressing them. This is the picture of the action of the lines
of force which Faraday gave me on the island of Arran, but I did
not understand it. In the picture the dotted curves are the Faraday
lines of force and the arrow-heads indicate the direction of the
electrical force. The Hertzian oscillator, and what Helmholtz had
told me before, made Faraday’s language and thoughts much more
intelligible. The work done by the machine is all expended upon the
stretching and compressing of the lines of force into the space
outside of the spheres, that is, upon the _electrification of that
space_.
Compare now the motion of the tuning-fork, after the pressure
of the fingers has been removed, to the electrical motion when
the air-gap has broken down and the action of the electrical
generator suspended. The prongs are driven back to their normal
position by the elastic reaction due to the bending; but when
they reach that position they are moving with a certain velocity,
and their momentum carries them beyond that position; they move
on until the energy of the moving mass has been expended in the
work of bending the prongs in the direction opposite to that of
the original bending. The prongs begin then to move back in the
opposite direction, starting the second cycle of motion. The same
line of reasoning will carry us into the third and fourth and
every succeeding cycle of motion. It is obvious that these cycles
will follow each other during equal intervals of time, which gives
a definite pitch to the tuning-fork. A periodic motion of this
type is called an oscillation or vibration; and it is clear that
it is a periodic transformation of the energy of elastic bending
into energy of motion of the mass of the prongs including the
surrounding air, and vice versa. The motion is finally reduced to
rest when the energy of bending, produced at the start by the work
of the fingers, has been used up. The question, what has become of
that energy? is very important in this connection. The answer is:
It is used up partly in overcoming internal friction and _partly
in overcoming the reactions of the surrounding air, which result
in sound-waves_. A sound-wave is a short name describing the
physical fact that in the air here are compressions and dilatations
alternating at periodically recurring intervals. The production
of sound-waves in the air is a proof that the air in the space
surrounding the tuning-fork participates in the motions of the
tuning-fork.
A perfectly analogous experiment was performed by Hertz with his
electrical oscillator, and his principal object was to find whether
the electrical field, that is, the electrified space surrounding
the oscillator, reacted as did the air driven by the vibrating
tuning-fork; if it did it would develop electrical waves. If these
electrical waves actually existed, what did Hertz expect them to
be? In the description of the oscillator and of its action, given
above, two things only were mentioned: the action of the electrical
machine which charges the oscillator and the reaction of the lines
of force against the tensions and pressures which crowd them into
the surrounding space. The electrical waves can, therefore, be
nothing else than periodic variations of the tensions and pressures
in the lines of force, that is to say, periodic variations in
the destiny of the lines of force in the space surrounding the
oscillator. This was what Hertz had found.
The breakdown of the air-gap in the electrical oscillator and the
consequent suspension of the action of the electrical generator is
analogous to the removing of the pressure of the fingers from the
prongs of the tuning-fork. The electrical charges on the spheres
with the lines of force attached to them, strained by tensions and
compressions, are released, and they move toward each other through
the conducting air-gap. Just as the prongs of the tuning-fork,
after the pressure of the fingers has been removed, cannot remain
in the strained position in which they have been bent, so the
electrical lines of force, after the insulating air-gap has broken
down and the action of the machine been suspended, cannot remain
in the position to which they are stretched; they contract, and
hence their positive terminals on one sphere and the negative on
the other move toward each other. The motion of the strained lines
of force with their terminals, the charges on the spheres, has a
momentum. Maxwell was the first to show that the momentum of the
moving electrical lines of force is equal to the number of magnetic
lines of force which, according to Oerstedt’s discovery, are
produced by the motion of the electrical lines of force.
The motion of the electrical lines of force has not only momentum
but also energy. Employing Faraday’s mode of expression we can say
that the electrical energy of the stretched electrical lines of
force is thus transformed into energy of the electrical motions.
This is perfectly analogous to the passage of the elastic energy
of the bent prongs of the tuning-fork into the energy of motion
of the moving mass of these prongs. Again, just as the momentum
of the moving mass of the tuning-fork bends the prongs in the
opposite direction and continues this bending until that motion
has disappeared, so the momentum of the moving electrical lines of
force will stretch again the electrical lines of force and continue
this stretching until the energy of motion has disappeared, when
the two spheres are charged again, but in the direction which is
opposite to that in the beginning. A new cycle of electrical motion
is then started again by the stretched electrical lines of force,
repeating itself in an oscillatory fashion until the original
electrical energy, produced by the charging electrical machine, has
disappeared.
But where has the energy gone? This question is just as important in
this case as it was in the case of the tuning-fork. The old electrical
theories answered this question one way, and Maxwell, inspired by
Faraday, answered it in another. The old theories maintained that
there is no other electrical motion except the motion of the charges
along the conducting surface of the spheres and the rods. They paid
no attention to the motion of the lines of force, because they knew
nothing about them. Their vision did not see the lines themselves
but only their terminals, the charges. Hence, according to the old
theories, all of the energy imparted by the machine is transformed into
heat in the conducting parts of the oscillator.
Hertz was the first to prove that a part of the energy is radiated
off into space, in a similar manner as the energy of a tuning-fork
is radiated off in the form of sound waves. He detected in the space
surrounding the oscillator the presence of electrical waves, that is,
periodically recurring variations of the density of the electrical
lines of force; he measured their length, and, having calculated
the period of his oscillator, he divided the wave-length by the
period and obtained the velocity of propagation. It came out, in his
earliest experiments, roughly equal to the velocity of light, as
the Faraday-Maxwell theory had predicted. The waves were reflected
and refracted by insulators denser than air, and all these and
other effects Hertz demonstrated to follow the laws which hold good
for light, supporting admirably Maxwell’s theory that light is an
electromagnetic disturbance. Even this preliminary report which Hertz
had sent to Helmholtz convinced everybody that the Faraday-Maxwell
electromagnetic theory had triumphed, and that our knowledge of
electromagnetic phenomena had been wonderfully extended. Subsequent
experiments by Hertz and others added more and more laurels to this
first victory.
That meeting of the Physical Society in Berlin was what I always
considered the inauguration day of the electromagnetic theory. Prior
to that day the theory existed in all its beautiful completeness, but
it dwelt on high in the celestial heights of Faraday and Maxwell.
Continental physicists needed the guidance of a Helmholtz to reach
these heights. After that day it came down to earth and lived among
mortal men and became part of their mode of thought. It was a heavenly
gift which Hertz brought down to earth. Everybody was convinced that
the science of light had become a part of the science of electricity.
_This new knowledge was the second great revelation of the nineteenth
century. The wonderful things which followed in its wake, even before
the nineteenth century had closed, testify to the greatness of that
revelation._
I have often asked myself the question, Why did not our Joseph Henry,
who discovered the oscillatory electrical motions and operated with
apparatus similar to that employed by Hertz, pursue his studies
further than he did in 1842? and why did not Maxwell, the formulator
of the modern electromagnetic science, perform those ideally simple
experiments which Hertz performed? The knowledge of the electrical
oscillator was the same in 1865 as in 1887, and Maxwell undoubtedly had
that knowledge. History offers an answer to these questions and this
answer throws a splendid light upon the character of these two great
scientists.
Soon after 1842 Joseph Henry resigned his professorship at Princeton
College, and bade good-by to his laboratory where he had made several
of his splendid discoveries, and where in 1832 he had constructed and
operated the first electromagnetic telegraph, one of the practical
results of his great discoveries. This happened long before Morse had
ever been heard of. Henry’s fame among men of science was very great
and promised to grow even greater if he continued his scientific
researches. He was still in his prime, only a few years over forty.
But a patriotic duty called him to Washington, where the Smithsonian
Institution waited for his skilled hand to organize it and to defend
it against the scheming politician. This duty tore him away from his
beloved laboratory, and he spent the rest of his life, over thirty
years, in Washington as secretary of the Smithsonian Institution,
as originator of most of the national scientific bureaus of which
this country is proud to-day. He was also the first president of
the National Academy of Sciences, chartered by Congress in 1863,
thanks to his efforts. Physical science under his leadership had
rendered valuable service to the country during the Civil War, and
the congressional charter to the National Academy of Sciences was a
graceful recognition of this service. I have already pointed out Joseph
Henry’s splendid efforts for the advancement of scientific research in
this country and shall return to it later. He was a great scientist,
but he was also a great patriot; his country stood first and his own
scientific achievements and fame stood second in his heart. That,
I am sure, was the reason why he did not pursue any further than he
did his researches of electrical oscillations. I will mention here
that one of the most gratifying results of my humble efforts was the
naming of an electrical unit after his name. My colleague, the late
Professor Francis Bacon Crocker of Columbia University, joined me most
enthusiastically in these efforts; and the Electrical Congress in
Chicago in 1893, at which Helmholtz presided, adopted the name Henry as
the unit of electrical inductance; the unit Farad was named in honor
of Faraday. No other electrical units are in more frequent use than
the Farad and the Henry, particularly in the radio art. No other men
contributed to this art as much as Faraday and Henry did.
Maxwell resigned his professorship at King’s College, London, at the
end of 1865, soon after he had communicated to the Royal Society his
great _memoir_ on the electromagnetic theory. The electromagnetic
theory of light which, as I pointed out before, he had called “great
guns” in a letter addressed to a friend, was the climax of it. He
retired to his country place, Glenlair, in Scotland, and for five years
he was free to devote his entire time to study and meditation. That
was the highest joy of his life. But the Duke of Devonshire, a loyal
Cambridge man, had presented the university with a goodly sum of money
for the building and equipment of a physical laboratory. It was to
be named the Cavendish laboratory, after Lord Cavendish, the Duke’s
illustrious ancestor, who had devoted his life to electrical science.
This gift was the Duke’s response to the Cambridge movement in favor
of scientific research. Maxwell was called to Cambridge to become the
director of the new laboratory, and he responded, knowing well that,
from that moment on, most of his time would be devoted to organization
and administration. Duty to his university, and to the cause of
scientific research in Great Britain, stood higher in his heart than
the experimental demonstration of his great theory; that was certainly
one of the reasons why Maxwell did not perform those ideally simple
experiments which Hertz performed. But as director of the Cavendish
laboratory he had trained a number of men, in order to prepare them to
push on the line of advance where he had left it; and one of them, in
particular, was soon to take the leadership in the rapid development of
the Faraday-Maxwell electromagnetic theory.
The examples of Henry and of Maxwell must have been in Andrew White’s
mind when in 1873 he spoke those memorable words which I quoted before
and will quote here again:
I will confine myself to the value, in our political progress, of
the spirit and example of some of the scientific workers of our
day and generation. What is the example which reveals that spirit?
It is an example of _zeal_, ... of _thoroughness_, of _bravery_,
... of _devotion to duty_ without which no scientific work can
be accomplished, ... of _faith_ that truth and goodness are
inseparable.
The Hertzian experiments created quite an upheaval in the research
programme of the Physical Institute; everybody seemed anxious to drop
his particular subject of research and try his hand at the Hertzian
waves. Several candidates for the doctor’s degree yielded, but I
resisted and returned to my problem in physical chemistry, and plodded
along as if nothing had happened. I was very anxious to finish my
research, get my doctor’s degree, and return to the United States.
But I soon found out that there are currents in human life which can
influence the course of life of a young scientist much more powerfully
than even a new and powerful current of thought in physical sciences.
During the first two months of 1888, Nikola, the Bosnian Serb, began to
look worried. He informed me confidentially that he had received bad
news about the health of his great “komshiya,” the aged Kaiser. The
audiences at the palace were separated by longer and longer intervals,
and Habel’s long table began to look deserted; the old generals with
their splendid uniforms were conspicuously absent and the historic
chop-house began to look commonplace. The daily parades of the guards
were finally suspended, and there were no expectant crowds in front of
the Imperial palace. The gay life of Unter den Linden became very much
subdued. Finally the historic event occurred: the great emperor died
on March 9, 1888. Berlin went into mourning and prepared for a funeral
such as Germany had never seen before. “I have secured a balcony for
you and your friends right over my store,” said Nikola; “I want you and
your friends to see the funeral procession as my guests.” His grief
over the death of the old emperor was really pathetic. He wanted me
and my American friends to see the great procession which, according
to his gloomy forebodings, was to mark the first step downward in the
wonderful development of the German Empire. When, consoling him, I
pointed out the well-known virtues of Crown Prince Frederick, he took
hold of his larynx and his gesture indicated that he expected the
death of the Crown Prince from his incurable malady. “What then?” I
asked him. He answered: “Ask your Bismarck and Moltke, Helmholtz and
Siemens; they are your oracles, perhaps they can answer your question;
no ordinary mortal can.”
Nikola had never met my American friends whom he mentioned in his
invitation, but he had heard a great deal about them. My classmate at
Columbia, A. V. Williams Jackson, now the distinguished Orientalist and
professor at Columbia University, was at that time at the University of
Halle, studying with the great Orientalist, Professor Geltner. He had
visited me in Berlin and I returned his visit by spending with him a
week-end at Halle. This was shortly before the great Kaiser’s death.
Jackson’s mother and two sisters were there on a longer visit, and for
two days I felt that I was back in New York again, and I was supremely
happy. On the way back to Berlin I could not dismiss from my mind the
memory of my mother’s words: “You must marry an American girl if you
wish to remain an American, which I know you do.” Ever since my return
from Halle, I could hear these words ringing in my ear no matter where
I was, in my lodgings, in the laboratory, in the lecture-rooms, or even
in Nikola’s store. Nikola had read my thoughts, and when he mentioned
my American friends he meant Jackson and his mother and sisters at
Halle. Well, they came, they saw, and they conquered. One of Jackson’s
sisters went to Italy during that spring and I followed; she returned
to Berlin to join her mother and I followed; she went to the island of
Norderney, in the North Sea, to spend a part of the summer season, and
I followed. The Faraday-Maxwell electromagnetic theory and the Hertzian
experiments, my research in physical chemistry, and the learned essays
of Helmholtz and Willard Gibbs, and of all the other fathers of
physical chemistry, disappeared from my mind as if they had never been
there. The only problem that could find a place there was the question:
Will she accept me? She finally did, and I made a bee-line for New
York, in order to find out how soon I could get a job there.
The Columbia authorities were organizing at that time a new department
in the School of Mines, the Department of Electrical Engineering, and
they were glad to see me and consult me about it. It was to start its
work a year from that time, that is, the end of September, 1889. I
was offered a position in it as “Teacher of Mathematical Physics in
the Department of Electrical Engineering.” A very long title, indeed,
but such it was and an interesting bit of history is attached to it.
I accepted gladly and hurried back to Europe proud as a peacock. My
fiancée and her family met me in London and I was married in the Greek
church, according to the rites of the Orthodox faith, the faith of my
mother and of all my ancestors.
“Marriage gives that fulness to life which nothing else can give,”
said Helmholtz when I saw him again in Berlin and informed him that
I was married and that I had been promised an academic position at
Columbia College. He approved my dropping the experimental research
and substituting in its place a mathematical research in physical
chemistry. This research was finished in the early spring and I sent
it to Helmholtz who was then in Baden-Baden. He telegraphed: “Your
successful effort approved and accepted.” Never before nor since did
I ever receive a telegram which made me more happy. The examinations
gave me no serious trouble, and in the late spring of that year I had
my doctor’s degree and became a citizen in the world of science. The
three theses which, according to old German custom, every candidate
seeking promotion to the dignity of a doctor of philosophy must frame
and defend publicly are given here, in order to show my final mental
attitude which was formulated by my scientific studies in Europe.
I. Instruction in Physics in the preparatory schools should be as much
as possible a practical one.
II. The Thermodynamic methods of Gibbs, von Helmholtz, and Planck form
the most reliable foundation for the study of those physical processes
which we cannot analyze by ordinary dynamics.
III. The Electromagnetic Theory of Light deserves more attention than
it has received so far in university lectures.
Usually these theses, appended to German doctor dissertations, are not
taken very seriously either by the candidate, who is to be promoted, or
by anybody else. But I took my theses very seriously. The first summed
up President Barnard’s doctrine relating to scientific instruction,
which I described before in connection with my description of the
American movement favoring scientific research in American colleges and
universities; the second summed up my admiration for the new science
of physical chemistry first started by our own Josiah Willard Gibbs;
and the third summed up my love for the Faraday-Maxwell electromagnetic
science. On these three questions in physical science I had, I thought,
quite clear and definite ideas; and that gave me much confidence that
I was about to return to the United States sufficiently equipped to
render service in return for some of the many favors which I had
received.