Perpetual motion machines are devices which operate under the principle of sustained or perpetual motion. That is, the machine will continue to perform a function repeatedly without stopping or requiring any type of human interaction. The motion is understood to take place by circumventing the various laws of thermodynamics and continuing to function as long as no outward influences interfered with the process.
In theory, perpetual motion can be understood to be in line with the science of physics. As an example, two objects in orbit around one another could conceivably remain in a perpetual state of orbit as long as no third object was introduced into the orbital cycle. At that point, the orbits would be impacted in some manner and could possibly alter the balance to the point that one object would slip out of the orbit.
A perpetual motion machine would attempt to either create energy from nothing or convert energy into work in some manner. Often, the idea of the water screw or siphon is held up as an example of a perpetual motion machine. However, in both cases, there is the need for some type of energy to trigger the phenomenon and some type of resources required to sustain the action. The ability to be self sustaining is not inherent to the design.
Still, many attempts have been made to create a perpetual motion machine that demonstrates the concept of sustaining motion. Basic plans to create a machine that sustains motion indefinitely can be found in writings reaching back to the 12th century. While the topic held a great deal of interest among scientists and academics for many centuries, there appears to have been waning interest after the dawn of the 20th century, as attention focused more on the creation of new technology to meet the needs of a changing world.
While the concept of a perpetual motion machine continues to be an item of casual interest, the idea of building such a machine tends to capture the attention of eccentrics and visionaries more than the scientists and engineers of today. From time to time, reports of the creation of a perpetual motion machine captures some media attention, then eventually fade into obscurity.