Methanol Cannon Demonstrations Revisited David A. ~olson,' Michael E. Dolson, Michael R. Hall, and Rubin Battino Wright State University, Dayton, OH 45435 Lisa S. June Northmont High School, Clayton, OH 45322 The methanol "cannon" is a popular demonstration. Generally, a few drops of methanol is placed in some suitable container that will not fragment when the methanol is ignited or cause harm if it hits someone. The ignition, depending on the design, is by match or tesla coil or a piezoelectric marker. The emohasis is on a vauor-uhase exdo. sive mixture simulating the explosion; occurring in a cur encine when a cvlinder is fil11:d with a fuel mixture from a carburetor. ~ y ~ & a l lpolyethylene y, bottles from the 50-mL to the 20-L size are used, the latter with special precautions! The electrodes are nails pushed through the sides of the containers about halfwav. UD. . In this pnper we describe two interesting variations. The first is a "chuin' rcaction using real metal chains. The second involves using easily avaiiable components to produce sequential explosions that can be musical in nature.
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The Chain Reaction In chemistry a chain reaction is one that proceeds in a series of steps with one reaction starting off the next, etc. Our chain reaction (modeled after the "Weird Science" group of Chicago) involves connecting six or more polyethylene bottles with -25-cm lengths of light chain terminating at both ends in miniature battery clips. The methanol (or ethanol or acetone) fuel is added to each bottle, corked, shaken. excess fuel is uoured off. and then the bottles are re-tuppcred. They wll rrmaln ~gnltabletbr at lens1 30 mm and so can be urenarrd ahcad oftme. The elwtrodcs in our 500- or 1000-ALbottles are nails that are pushed through a t about the half-height level and adiusted to have a 1-2 mni gap. The most lmpurtant h n g about connwtlng up thc bottles wrth t h l~m c ~ h sof cha~nIS that thv cham segments (you can always talk about catenaries) are not lying on the table top, but actually are suspended. Otherwise, electrical contact is poor. If you use a tesla coil, then remember to ground the last bottle to a ring stand using a chain segment. If you use a piezoelectric sparker, you need to connect i t to both ends of the series of bottles and that may involve quite long leads or an artful arrangement to keep the chains suspended. Ignition generally sets off all bottles at the same time. Despite our best efforts, a bottle or two may not ignite. Using the car engine analogy, we can point out that our engine needs a "tune-up"! For more interesting "chain" reactions, you can alternate bottle sizes or arrange different sizes in interesting patterns. Students enjoy the sophomoric pun involved in this "chain" reaction. Sequential Explosions Because i t would he a n interesting demonstration to have the methanol cannons explode in sequence, we explored various methods of doing this. Ordinary RC time constant time delay circuits with the high voltage supplied by an automobile ignition coil did not work. However, working with the coil gave us the idea of simply adapting 'Author to whom correspondence should be addressed
732
Journal of Chemical Education
Figure 1. The sequential firing apparatus an automobile ignition system. We have done this quite successfully and the details follow. The power supply may be a 12 V motorcycle or car hattery, or a 12 V power supply of 4 Aminimum capacity, or a 12 V battery charger. All work quite well. You can obtain an eight-cylinder car distributor from a nsed parts lot or buy one new for -550. ('helve-cylinder distributors are very expensive.) You can find a nsed coil or buy one new for -$20. Yon can buy distributor wire and connectors and make your own wire harness or buy a new eight-wire assembly for -515. We found that you can "solder" copper alligator clips into the ends of the wire connectors to make both a good mechanical and electrical connection. Areturn electrical bus may be made by soldering 25-cm leads ending in alligator clips to a heavy length of copper wire or any other suitable conductor. This bus should be grounded. We nm the ground to the prong of a three-wire plug that has no other wires and simply plug this into an outlet. Depending on your site, other ways of grounding may be nsed. The simplest and least expensive low-speed motor we could find was a replacement motor for a home barbecue grill. We bought two different versions a t -520 each; one ran on 120 V AC and other on two D batteries. The distributor was fastened securely to a ring stand using two three-finger clamps. The motors were mounted to a 4-in. ring using nylon cable ties. The couplings between the motors and the distributor were fabricated in our machine shop because one motor took a square and the other a round shaft with a flat. Couplings may be made by adapting the hardware that comes with the motors. For example, we cut off the round shaft near its threaded end and fastened a U-shaped piece of iron to i t to mate to the distributor with nuts and a lock washer. Because there are so many different styles of motors and ways to connect to the
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Spark Gaps Center Terminal
"Mary Had A Little Lamb" with the following sequence of unstoppered bottles: 250, 500, 2000,1000,250,250,250 mL. The bottles may be left unstoppered for a different and quieter demonstration, but they need to be "fired" soon after loading with the fuel mixture. Remember to remove the "spent" gases in the bottles between uses by filling completely with water and emptying or using vacuum. Finally, because this is such a popular demonstration we measured peak sound levels in a large lecture hall a t distances of 3 and 5 m for 60 mL, 125 mL, 250 mL, 500 mL, 1L, 2 L, 4 L, and 20 L for bottles sizes, all usingmethano1 vapor and all being tightly corked. The averaae peak level a t 3 m for all of the bottles wasli9.3 db (s = 3.6) and 124.0 db ( s = 3.2) for 5 m. I t is interesting that the peak level was essentially the samefor all containers and differed only slightly with distance. We also tested the 1000-mL bottles using acetone and ethanol. There are no differences in the sound levels. We found no difference in handling between the solvents. Pedagogically, you can do these demonstrations as sheer fun, illustrations of combustion, or a s a n illustration of how a car engine works. If all you wish to do is indicate the firing sequence, this can be done with a suitably arranged set of nails driven into a board. When we recently demonstrated the sequential explosions, a n anonymous high school teacher told us she had achieved similar results using nails in a board, appropriately connecting the nails to the bottles and the bottles to a common ground, and simply moving a t e d a coil down the line. Quite ingenious and well-worth trying. We have developed a n inexpensive tesla coil based on a car coil (1).
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Figure 2. Wiring diagram for apparatus. distributor, we must lea\.e the details of that coupling to each individual. To slow the rotation of the distnbutor we used timing pulleys and belts (available from Small Parts, Inc., P.O. Box 4650, Miami Lakes, FL 33014) adapted to the apparatus by our machine shop. The distributor should turn in the range of 2-12 rpm depending on your demonstration. Firmre 1is a drawing of our a ~ ~ a r a t u s . 2. All temporary The wirigg diagram is && in ~i~uz connections were made with alligator or small battery clips. Caution
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Be sure to have a good ground and to test leakage to ground if you are using a power supply. Mark the shaft of the distributor to indicate where the mov. ing contact is. It also h e l ~ to s number the wires in the harness. It would be a good safety precaution to be able to turn every thing on and offat same distance by a switchedextension box with its own circuit breaker.
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There are several demonstrations that can be done with this device. The first is simply to use polyethylene bottles of the same size like 500 or 1000 mL. A second is to do the series 50, 100,250,500, 1000,2000, [pause], 20,000 mL. A third is to mix the sizes randomly. Our favorite is to try for "musical" explosions. We attained the first seven notes of
Acknowledgment
We wish to thank J. A. Arehart and S. Brockman for their assistance and NSF for support for LSJ as a summer research apprentice, Literature Cited I.
H ~ IM..R.; ~ ~ t t i R. "~ in .press.
Volume 72 Number 8 August 1995
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