What do you call this mechanical device?

Solution 1:

This is called a Rube Goldberg machine.

From Wikipedia:

A Rube Goldberg machine is a contraption, invention, device or apparatus that is deliberately over-engineered or overdone to perform a very simple task in a very complicated fashion, usually including a chain reaction. The expression is named after American cartoonist and inventor Rube Goldberg (1883–1970).

An example is Goldberg's "Professor Butts and the Self-Operating Napkin", which was later reprinted in a few book collections, including the postcard book Rube Goldberg's Inventions! and the hardcover Rube Goldberg: Inventions, both compiled by Maynard Frank Wolfe from the Rube Goldberg Archives.[3] The "Self-Operating Napkin" is activated when soup spoon (A) is raised to mouth, pulling string (B) and thereby jerking ladle (C), which throws cracker (D) past parrot (E). Parrot jumps after cracker and perch (F) tilts, upsetting seeds (G) into pail (H). Extra weight in pail pulls cord (I), which opens and lights automatic cigar lighter (J), setting off skyrocket (K) which causes sickle (L) to cut string (M) and allow pendulum with attached napkin to swing back and forth, thereby wiping chin.

And here's one of my favorites: The machine in this Honda Commercial is all made from Honda car parts and demonstrates a high level of finesse.

Solution 2:

I think you are referring to a chain reaction machine:

• A Chain Reaction Machine uses natural forces, like gravity and elasticity, to make something happen. The chain is a series of simple devices like a pulley or some dominoes that knock into each other. The idea is to put together a few of these devices so that they go off one right after the other, like a chain reaction. Once you put the first one in motion, the rest of the machine should go by itself.