About Kinetic Mobiles

(Shortcuts to mobile history, mechanics, construction.) A mobile is a hanging sculpture of moving parts that twist and sway in the breeze. Mobiles are very light, skeletal constructions, often made of wires and plates that seem to float in space. They are thought of as abstract scupture, but this is only because it is very, very hard to make one that actually looks like something.

A little history

Mobiles are most famously associated with Alexander Calder, third in a family line of sculptors, who began experimenting with hanging wire constructions in the late 1920's. Credit for the invention may actually be due to Naum Gabo, the Russian constructivist artist who made articulated hanging constructions earlier in the decade. Gabo was exploring a new idea in art: Sculptures that continually change their appearance, but are constant in what they represent. It is Calder who fully developed the idea of a mobile as a kinetic assemblage that sculpts space. Calder's early kinetic sculptures were abstract motorized constructions, plays on mechanical models of the solar system whose planets and moons would carve circles and cycloids out of space as they whirled about each each other. To these Marcel Duchamp gave the name "mobiles" in 1931. Soon after, Calder abandoned motors and began exploring wind-powered mobiles, building very delicate wire sculptures tipped with metal sails to catch the breeze.

Mobile mechanics

Mechanically, a mobile is a cascade of levers. Each lever is suspended from above, and has masses or other levers suspended from its ends.

In this particular mobile, each lever is formed of a single piece of metal bent into a loop and hook at exactly the right place for balance. The fulcrum (loop) is located so that it balances the counterweight, the weight of the wire, and the weight of the sub-mobile that hangs from the hook.

The hook-and-loop construction makes the mobile behave like a spring when it twists. When a loop twists in its hook, it climbs up the sides, converting the mobile's kinetic energy into potential energy as it hoists itself up. Some cleverly designed mobiles use this to control how energetically parts move relative to each other. The hook-and-loop construction also moves the fulcrum above the rest of the wire, which makes it possible to balance the lever at a skewed angle. (This is why rod-and-string mobiles are always horizontal and dull.) Hook-and-loop looks interesting, but it is often hard to get the balances right.

Mobile construction

Mobiles are a difficult medium. It can be quite hard to make a mobile look "as intended." Where the masses will float in space, how long the wires are, and where the fulcrums wind up are largely dictated by the mechanics of balance. This is probably why nearly all mobiles are abstract and not representational.

Inside-out construction

Calder said he usually meant to make one thing and wound up making another.

He built his mobiles from the inside out, first looping and hooking a wire, then looking for a spot along its length where a weight would balance. If you go to a museum and you can see traces of his trial-and-error method in the way he clipped wires and stapled on weights where ever he found a favorable balance. Occasionally he built two or three models of the same mobile, trying to find a set of balance points that would allow him to place the weights exactly where he wanted them to float in space.

Outside-in construction

Alternatively, you can build a mobile from the outside in: Choose where the weights will float, sketch wires, estimate balance points, and repeatedly revise the wires until they pass through their balance points. Because the wires have mass, revisions change the balance points, making outside-in method rather painstaking. However, because it allows much more control over the what the mobile will actually look like, it opens the door to a realm of much more expressive and representational kinetic sculptures. For example, here is a
mobile bestiary I made this way.

Although painstaking by hand, prebalancing is perfectly suited to a computer artist capable of precise calculations. Of course, this is the only part of mobile design that computers can do with ease. To see how a computer was endowed with the vision, design, and aesthetic sense to do the rest, visit the artificial artist page.

Matthew Brand / MIT Media Lab / brand@media.mit.edu