| "Ex nihilo nihil" Anaxagoras, Greek philosopher 500 B.C. |
"Perpetual Motion? What a foolish notion! Only cranks and crackpots think such a thing is possible; only fools are interested in such things." That's the reaction of many people, and most scientists. But let's consider the matter by looking at the long history of this subject and its relation to science.
The notion of perpetual motion is interesting from the perspective of history of science and technology, for it illustrates both the search for a perfect machine and the development of our modern understanding of the energy principle in physics. Besides, perpetual motion machines and their inventors provide a case study of human psychology: ingenuity, persistence, optimism and fanatism, even in the face of repeated failures.
A perpetual motion machine (PMM) is a device based on mechanical, chemical, electrical or other physical processes which, when started, will remain in operation forever and provide additional work as well. Only the natural wear of the components will eventually stop its operation. Consider this definition, written in 1896!
| Definition from Meyers Enzyklopädie: "[...] in general a thing that is perpetually in motion. Each part which is set into motion must continue in motion so long as external forces do not hinder it. A pendulum would swing forever were it not for air resistance which reduces its kinetic energy. [...] Since friction is always present, a machine which operates cyclically without external energy input belongs to the realm of the impossible.1 (vol.13 p.680 l.col.) |
This definition is incomplete, for it restricts the concept to purely mechanical devices. The Enzyklopädie Naturwissenschaft und Technik from 1981 is more precise and clearly distinguishes between two types of Perpetua Mobilia:
| "Perpetual motion machine of the first kind.. By this we
mean a device whose parts are not only in permanent motion, but moreover
is able to provide useful work without input of external energy (e.g. warmth)
and without change of the physical or chemical status of its parts. A perpetual
motion mahine of the first kind does not exist. It would contradict the first
law of thermodynamics"
(vol.4, p.3236, right col.)2
"Perpetual motion machine of the second kind. By this we mean a machine undergoing a cyclic process which does nothing more than convert warmth into mechanical (or other) work. A perpetual motion machine of the second kind does not exist. It would contradict the second law of thermodynamics." (vol.4, p.3236, right col.)3 |
I'm not completely satisfied with these definitions. Although they clearly state the reason why a PM device does not work, they tell us nothing about the structure and the intended principle of operation of the specific machines.
Besides mechanical energy sources, a PM device may use other forms of energy. Magnetic, electric or thermodynamic processes are often used. I will return to this in the appropriate chapter.
The early literature does not use the modern definitions, but instead it distinguishes between natural perpetual motion (perpetuum mobile naturae) and "physical" i.e. artificial perpetual motion (perpetuum mobile physicae). In nature the motion of the sun and stars in the celestial sphere, and the recurring solstices "proved" that eternal, or perpetual, motion is possible. These phenomena suggested the appealing conclusion that man might reproduce God's creation in smaller scale. But this carried risk, for perpetual motion seekers were regarded as heretics, and potential victims of the inquisition. On the other hand, the idea of perpetual motion was an ambitious idea which promised to have great value, and this provided some security. The Jesuits, the scientists of the church, worked diligently on perpetual motion projects. This leads us directly to the heart of the history of a very special kind of machine. This is how the whole story began...
1. Original German text: "[...] im
allgemeinen ein Ding, das sich fortwährend bewegt. Jeder Körper
muß, wenn er einmal in Bewegung gesetzt ist, in derselben verharren,
solang ihn nicht äußere Umstände daran hindern. Ein Pendel
würde unaufhörlich schwingen, wenn nicht die Reibung der Luft die
lebendige Kraft allmählich aufzehrten [...] Da nun aber die erwähnten
Hindernisse überall und immer sich geltend machen, so gehört eine
Maschine, welche sich ohne Energiezufluß von außen fort und fort
bewegt, zu den Unmöglichkeiten." (13.Bd. S.680 li.Sp.)
2. Original German text: "Perpetuum
mobile erster Art. Unter einem Perpetuum mobile erster Art versteht
man eine Vorrichtung, deren Teile nicht nur dauernd in Bewegung bleiben,
sondern die sogar dauernd Arbeit zu leisten vermag, ohne daß von
außen Energie (z.B. in Form von Wärme) zugeführt wird, ohne
daß sich aber auch der physikalische oder chemische Zustand der an
der Vorrichtung beteiligten Stoffe mit der Zeit ändert. Ein Perpetuum
mobile erster Art gibt es nicht. Es würde im Widerspruch zum ersten
Hauptsatz der Thermodynamik stehen." (4.Bd. S.3236, re.Sp.)
3. Original German text:
"Perpetuum mobile zweiter Art. Unter einem Perpetuum
mobile zweiter Art versteht man eine periodisch arbeitende Maschine, die
nichts anderes tut, als Wärme in mechanische (oder eine andere) Arbeit
zu verwandeln. Ein Perpetuum Mobile zweiter Art gibt es nicht. Es würde
im Widerspruch zum zweiten Hauptsatz der Thermodynamik stehen."
(4.Bd. S.3236, re.Sp.)
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