Tricks of the trade

Ian B Dunne

Ian B Dunne explains how to weave a little magic into experiments.

Getting awe and wonder into the classroom is never easy, but what about using a little magic? There is a long history of using magic tricks to get children interested in science. In 1832 - the same year as Michael Faraday invented the electric generator and Charles Darwin sailed off on HMS Beagle - Sir David Brewster wrote Letters on Natural Magic, a compendium of science tricks. In those days physics was called natural philosophy, biology was natural history and chemistry was called chymistry. Some of Sir David Brewster's material is usable today, but much is not. Sadly, we cannot use poisonous mercury to make disappearing statues any more - not without giving the nice folks from CLEAPSS School Science Service palpitations!

Some tricks need specialised equipment, but many others need equipment which tends to lie around the classroom.

Conjuring tricks in the classroom can achieve several things. First, many tricks rely on simply fooling the audience into looking the wrong way or using some other equally simple method to dupe them. This can be used to teach important lessons about observation.

A splendid story tells how a consultant shows medical students how to test for diabetes. He plunges his finger into a urine specimen and then into his mouth. He tells the students to do the same, which they do to their great distaste. Then he says that with experience you can taste the sugars, finally adding that if they had been watching properly they would have noticed that he put his middle finger into the sample, but his forefinger into his mouth. Moral: learn the value of close observation. Observation is included all the way through the science curriculum, starting at Sc1 1A.

Tricks can also be used as "betchas" to get pupils' attention and challenge preconceived ideas, as in "I bet you I can turn this bucket of water upside down and not spill a drop". To do this, take a bucket of water with a rope tied to the handle and have courage. If you swing it in a smooth circle over your head, the water will stay in it. This experiment fits into key stage 2 6E Forces in Action, or Sc4.2 Forces and Motion.

Some magic tricks demonstrate scientific principles. If teaching Sc4.2 Forces and Motion at any key stage, why not start off talking about inertia by performing the old favourite of pulling a table cloth off a laid table? A classroom table, a sheet and some books or other objects will do; just remember to pull the table cloth quickly and smoothly.

Alternatively, to show action, reaction and inertia, place a long thin stick a metre long on top of two plastic cups - the cups on chairs and the stick like a bridge between the chairs. Now, with a cricket or baseball bat, hit the stick hard in the middle. The stick will break and the cups will be fine. This trick used to be performed at fairs and much money would change hands as bets were made.

The simplest initial "betcha" is to place a coin on a card, put the card on your finger and bet the audience that you can flick the card out of the way and leave the coin unmoved.

Demonstration and "magic" principles can also be applied to air pressure and changing state (module 5D), Sc3.2. An example is putting a hard-boiled egg into a bottle. You need a glass bottle with a neck a little smaller than the diameter of a shelled hard-boiled egg. Half a sheet of A4 is placed into the bottle, a lit match is dropped in and the egg placed into the neck of the bottle. The egg will bump up and down a little as the air heats and then will be inexorably drawn into the bottle, finally dropping in with a satisfying plop. The burning paper has used some of the oxygen in the air, as well as heating the air and expelling a little. Air then cools and reduces in volume. Pressure of the atmosphere forces the egg into the bottle. Extracting the egg again is not part of the demonstration!

Another historic demonstration involves placing a large tin can containing a little water on to a Bunsen burner and heating it until it steams. Next, put the lid on, turn the heat off and stand back. As the steam condenses it reduces in volume by a couple of orders of magnitude and the weight of 32 kilometres of air above the can crushes it.

Luckily, modern materials greatly simplify (and improve the safety of) this experiment. It is relevant to Sc3.2 Changing Materials and impressive at any key stage, but a safer way of doing it is like this: use an empty three-litre fizzy-drink bottle, boil a kettle and pour 250 millilitres of water into the bottle. Give it a little shake, put the lid on and leave it on the table. The bottle will collapse with some very satisfying noises until after about 15 minutes it is crushed. As with all things, practice makes perfect, so try each trick at home before you try any in front of a class. It is worth it for the look on their faces.

Resources

Books are the best resources. The first two items are out of print, but are on microfiche in many university libraries and are useful for cross-curricular links to history, geography and numeracy.

Chymical, Natural, and Physical Magic: Intended for the Instruction and Entertainment of Juveniles During the Holiday Vacation

By Septimus Piesse (1865). Letters on Natural Magic. By Sir David Brewster (1832). Particularly useful books include: Secrets of 123 Classic Science Tricks and Experiments. By Edi Lanners. McGraw-Hill Education, pound;12.95

Magical Science: Magical Tricks for Young Scientists. By Eric Ladizinsky. Lowell House, pound;2.75

* For a range of books on a similar theme visit Dover Publishing at www.store.doverpublications.com

Ian B Dunne is a scientist and educator based in Hampshire. He travels around the country presenting shows such as Science ... the Best Bits, and also visits schools to inspire pupils and teachers about science and the world around them www.ianbdunne.co.uk

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