An Inexpensive Ignition Device

A number of demonstrations involving the ignition of explosive mixtures of gases require the use of a high-voltage source, such as a Tesla coil or an induction cod. An alternative ignition device is the push-button ignition unit used on many gas barbecues. With a few minor modifications, it may be used to provide a safe, inexpensive source of moderately high potential, producing a spark acmss a 5-mm gap.

The igniter (salvaged from an old barbecue or purchased from an appliance repair dealer for under $10) is based on the piezoelectric effect. It contains a special crystal that, when given a sharp blow, becomes momentarily distorted, setting up a high potential across the crystal faces. To make the unit easier to use, two 20-em lengths of insulated electrical wire (14-gauge) were soldered to the leads of the igniter, fastened to the body of the igniter using hot melt glue so that they project out from the end ofthe igniter, and then covered with electrical tape. The ends of the two wires were stripped and bent to provide a narrow gap. When the button of the igniter is pressed a stmng spark jumps across the gap.

This modified igniter may be used to light a Bunsen burner or ignite a mixture of gases in an open container. By bending the ends of the wires apart and using wires with alligator clips to connect the igniter to the electrodes of a eudiomiter tube, the igniter ma; be used to ignite a mixture ofhydrogin and oxygen.

I have found thm igniter to be an excellent device ta ignite a mixture of air and methanol. Two nailr are msened through the sides of a 500-mL plastic shampoo bottle a larger bottle is not recommended for safety reasons) near the bottom so that there is about a 5-mm gap between the ends of the nails. After adding a small quantity (1-2 mL) of methanol to the bottle, corking the bottle, and shaking i t vigorously to vaporize the methanol, the bottle is clamped to a stand and pointed in a safe direction. The igniter is then connected to the nails. To make the demonstration more dramatic, I begin a count-down with the students and, when zero is reached, I press the button. The cork is shot with a terrific hang acmss the room. To f r e it once again, I squeeze the bottle several times (to remove the products of combustion), replace the cork, and shake again. There is usually enough methanol left for several explosions. If the rwm can be darkened. leaving the bottle uncorked allows the students to see a beautiful flame shoot out of the bottle when the mixture is ignited. -

This igniter gives much better control over the ignition process than the Tesla coil, it is certainly much cheaper, and, for those somewhat hesitant about being close to an explosive mixture, longer wires can easily be used to connect the - igniter to the electrodes.

Henry Brouwer Redeemer College,

Ancaster, ON. Canada, L9G 3N6

Volume 70 Number 4 April 1993 329