From Wikipedia, the free encyclopedia

A bed of nails is typically an oblong piece of wood, the size of a bed, with nails pointing upwards out of it. It appears to the spectator that anyone lying on this "bed" would be injured by the nails, but this is not so, assuming the nails are numerous enough, since the weight is distributed between them such that the force exerted on each nail is not enough to break the person's skin.

One use of such a device is for magic tricks or physics demonstrations. A famous example requires a volunteer to lie on a bed of several thousand nails, with a board on top of him. Cinder blocks are placed on the board, and then smashed with a sledgehammer. Despite the seemingly unavoidable force, the volunteer is not harmed: the force from the blow is spread among the thousands of nails, and the breaking of the blocks also dissipates much of the energy from the hammer.

This demonstration of the principles of weight distribution requires that the weight of the volunteer be spread over as many nails as possible. The most dangerous part is the moment of lying down or getting up, when one's weight may briefly be supported on only a few nails. Some "beds" have rails mounted at the sides to help users lie down and get up safely.

The bed of nails is also used by some for meditation.

In electronics, "bed of nails" can also refer to a type of device used to connect a printed circuit board device under test (DUT) to automatic test equipment. Named by analogy with a real-world bed of nails, these devices contain an array of small, spring-loaded pogo pins; each pogo pin makes contact with one node in the circuitry of the DUT. By pressing the DUT down against the bed of nails, reliable contact can be quickly, simultaneously made with hundreds or even thousands of inidividual test points within the circuitry of the DUT. The hold-down force may be provided mechanically or by means of a vacuum pulling the DUT downwards onto the nails.

Devices that have been tested on a bed of nails tester may show evidence of this after the fact: small dimples (from the sharp tips of the pogo pins) can often be seen on many of the soldered connections of the PC board.

• Australian Skeptics -- how the bed of nails works and how to make one
• DarylScience's E-World -- the bed of nails in use as a school physics demonstration