How do they do it?

I have long been a firm believer that to teach anything effectively, you have to first get the audience's attention and, preferably, their interest. This belief stems from a personal experience I had early in my physics career. For the first year that I studied physics, at an English grammar school, my teacher regularly stood up in front of the class and talked to, or rather at, the students for about fifty minutes, nonstop. I soon got very bored with this class, coming close to failing it. The next year, because of a move brought on by a change in my father's career, I attended a different school. My new physics teacher made liberal use of demonstrations when he taught, often with a dramatic flair. This got our attention. This made physics fun, exciting, and real to us. We could see what it was being applied to. I've been interested in physics ever since, and especially in dramatic demonstrations. In this article I describe the physics behind four of the more dramatic demonstrations that I now do on occasion for my physics classes: walking on a bed of broken glass, having a concrete block broken on me while lying sandwiched between two beds of nails, dipping my fingers in molten lead, and picking up an orange-hot piece of space shuttle tile.

Walking on Broken Glass

For this demonstration the glass bottles should first be soaked in water to remove any paper labels. An alternative is to use Mason jars. It is best to use fairly large bottles so that the pieces formed will have only a gentle curve to them. When breaking the bottles I place them in a canvas sack and use a hammer, being sure to wear gloves and eye protection. The glass should be broken into fairly small pieces. The bed for the glass may be made from half-inch-thick plywood framed by pieces of 2"34" wood. Once the glass has been poured into the bed it should be spread out to a uniform depth. Any piece that has a right-angled bend in it, where the sidewall of the bottle meets the base, is moved to the edges of the bed so that only relatively flat pieces of glass are included in the center of the bed where the walking takes place. As an extra precaution, I cover the glass with a cloth and then use a large cast iron skillet to pound the surface firmly. This ensures no points of glass are sticking up. This is usually done before the audience enters the room. A bed of glass about 8 cm (three inches) deep seems best, as this provides sufficient depth for the glass to be able to shift and settle somewhat as a foot is planted slowly and directly down upon it. When done this way the pieces of glass lay fairly flat and no edge presses perpendicularly against the sole. The bottom of the foot has some give to it and conforms to the shallow curve of the glass pieces. This is similar to a sharp knife being pressed with the flat of the blade against one's flesh, where considerable force may be used without injury. When walking I place each foot slowly, moving it elsewhere if a point or edge is felt, although that is seldom necessary if the bed has been prepared correctly. Care must be taken to brush off any pieces of glass that stick to the bottom of the feet when stepping off the bed.

To show that the edges of the glass are sharp I use a piece from the bed to cut the string suspending a bowling ball about a half meter (two feet) above the floor.

Dipping Fingers in Molten Lead

Dipping one's fingers in molten lead is usually cited as being an example of the Leidenfrost effect in action. However, it is not quite the same situation as when a drop of water is lifted up and hence somewhat insulated from a very hot skillet by the steam formed beneath it. Before dipping one's fingers in molten lead, the hand is dipped in a bowl of water. Then the drops are shaken off and the hand dipped quickly in and out of the lead. I usually dip the first seven or eight centimeters of my fingers. Heat from the lead goes into evaporating the water and hence not into burning the hand, and the resulting steam layer insulates the hand.

If we suppose that the fingers of one hand have a total surface area of about 100 square cm, then a layer of water 0.1 mm thick would require 2600 J of heat to warm it from 20°C to 100°C and then to boil it at 100°C. This would mean that even if the lead were to cool by 200°C, the amount cooled would need to be 100 grams, which would entail a layer almost a millimeter in thickness. The specific heat of lead is relatively small when compared to the specific heat-and more importantly the latent heat-of water.

It is very important when doing this demonstration that the lead be heated well above its melting point of 327.5°C. Having lead solidify on one's fingers is not pleasant. When the molten lead is hot enough, about 200°C hotter than its melting point, a gold-colored film will form on its surface. It is a good idea to have well-trimmed fingernails so that no lead gets beneath them, although I have seen the demonstration done twice by women with long painted fingernails, apparently without damage or injury. Should the lead contain impurities, or for other reasons form a dross on the surface, this should be cleaned off using a spoon fastened to a wooden handle. I find that the density of the lead makes a stronger impression on those who perform this demonstration than its temperature. Remember that lead should only be melted where there is adequate ventilation.

Bed of Nails and Breaking a Concrete Block

Having a concrete block broken on your chest while lying sandwiched between two beds of nails usually elicits three different questions: Why one can lie on the bed of nails, what happens to the kinetic energy of the hammer, and what happens to the momentum of the hammer? The reason that one can lie on the bed of nails has to do with the difference between force and pressure. The nails are spaced about two centimeters apart, and hence when I lie flat a sufficient number of them support me such that no one nail has to press on my body particularly hard. I would estimate that for the bed of nails I use, made from a thousand nails, at least 150 nails support my 150 lb. weight. So even if my weight is not quite evenly distributed, no nail pushes on me with much more than 2.5 lbs. of force and that is not uncomfortable. The bed I use is made from aluminum gutter spikes, and as purchased the points of these are somewhat blunted. This is done by the manufacturer so that the spikes will not split a wooden board if driven into one. This does make for a more comfortable bed than if sharper steel nails are used. Particular care needs to be taken when getting on and off the bed, and I have seen side rails added to a bed to aid in this procedure.

When the concrete block is broken, the kinetic energy of the sledgehammer goes into causing the block's destruction, ultimately warming the pieces, and the momentum of the hammer is passed through the prone person to the earth. The person swinging the sledge hammer needs to hit the block with sufficient force to shatter it, but not so hard that the hammer has a significant amount of energy left after the initial impact. It is quite possible for an adult to hit the block too hard, as I found out when an enthusiastic and strong gym teacher hit a block as hard as he could. All the breath was knocked from me and I had a matrix of puncture wounds on my chest and back. A tetanus shot saw me fine that day, and since then only my wife breaks the block. She practiced just breaking blocks on the ground many times before breaking them on me. Three-section blocks should be placed lengthwise on the top board whereas two-section blocks are best stood on end. I prefer two- section blocks as they shatter nicely.

The small pieces fly with force and must be guarded against. Also, the end pieces of the concrete block do tend to fall off the top bed of nails. Hence it is advisable to wear a face mask as well as safety goggles, hold your forearms over your face, and have other protection, such as a board placed over any other exposed areas.

Picking Up a Piece of Orange-hot Space Tile

The hot tile demonstration illustrates the difference between temperature and heat. Small sample pieces of tile from the space shuttle are occasionally obtainable from the Lockheed Missiles and Space Company. If such a piece is heated using a Bunsen burner or propane torch until it is orange hot (about 1,000°C), it may be picked up by the edges a few seconds after the heat source is removed.

Several factors contribute to why this may be done. The basic raw material for the tile is 99.7% pure silica, which has low heat capacity and an extremely small thermal conductivity (approximately 5 J/cm.hr.°C.) Holding the tile by the edges puts the fingers in contact with the part of the tile that has cooled the most in the few seconds since the tile was heated. It also provides only a small area of contact, which can be changed if the tile is to be held for more than a few seconds.

An analogy may be made between heat flowing from the tile and water flowing from a hose pipe. Although the temperature difference between the tile and the hand, analogous to the pressure difference between inside and outside the hose, may be large, if the cross section of the hose is very small, analogous to the very small thermal conductivity of the tile, then the rate of water flow from the hose, analogous to the flow of heat from the tile to the hand, will still only be small. A small piece of tile seen from close up works best, as it is very difficult to heat a large piece uniformly unless the heating is done in an oven capable of temperatures in excess of 1,000°C. If a suitable oven, such as a ceramics kiln, is available, a dramatic photograph may be taken of a person holding a piece of the tile in a darkened room, the only illumination for the picture being the tile's incandescence. The dead layer of skin on the fingertips is generally fairly thick, on the order of half a millimeter or so, and is also a poor heat conductor. Many of the same principles apply to someone firewalking, where the feet, ash, and coals are poor conductors and the time of contact is short.

These demonstrations can be dangerous; do not try them at home. If due care is exercised, however, each is relatively harmless and guaranteed to grab attention.