72 SCHOOL SCIENCE AND MATHEMATICS

UNUSUAL DEMONSTRATION EXPERIMENTS

BY J. 0. FRANKState Teachers College, Oshkosh, Wisconsin

AND

GUY J. BARLOWPrincipal McKinley Junior High School, Applelon, Wisconsin

The lecture demonstration is recognized as a very importantphase of science teaching, yet comparatively little has beenwritten describing its technique. The demonstration is ordina-rily thought of as an experiment carried out by the instructorin the classroom before the class. Explanation, questions orother remarks may or may not accompany the actual experi-ment. Other methods of using demonstration experiments havebut rarely been described in the literature of science teaching.

In the following pages an unusual method of using the demon-stration experiment is described and experiments suited to themethod are given.

PROBLEM AND MYSTERY DEMONSTRATIONS

One method of inducing interest in a new phase of science isto prove to the student that it affords explanation of facts orphenomena not ordinarily understood. Merely telling the stu-dents this does not suffice, as the "Oh Yeah" attitude is oftenthe result. But when the teacher can show something which atfirst looks to be very ordinary but which on further investiga-tion cannot be easily explained�the student is usually inter-ested. And some students will be so intrigued by the problemoffered that they will be driven to study, experimentation andother extra effort�until a satisfactory solution is found. Anunsolved problem which is only an unsolved problem offers littlechallenge�the student may simply dismiss it with the thought^this is not in my field�it is a problem for somebody else tosolve." But an unsolved problem, which has to do with unusualaspects of ordinary things, which offers something hither-to un-noticed about familiar objects or materials�that arouses inter-est at once. On this fact is based much of the interest in detec-tive stories and in puzzles (crossword and otherwise). Letus illustrate:-

DEMONSTRATION EXPERIMENTS 73

THE SWINGING WATCH

Wliatis Seen:A watchishungonasmall

nail in such a way as to befree to move sidewise. Stu-dents are asked to observecarefully what happens.The watch begins to swaywith a pendulum-like mo-tion and with an ampli-tude which increases to amaximum and then decreases until no motion is seen. Then thecycle is repeated.

Explanation:The watch acts as a simple pendulum whose oscillations are

caused by the balance wheel which acts as a secondary pendu-lum.

In using this experiment the teacher may simply hang thewatch on a small nail and ask the pupils independently to note,record and explain what happens. After several minutes of ob-servation the watch will be removed and a pupil asked to writeon the board briefly but exactly what happened. Other pupilswill correct and extend the record of the facts observed. (Greatdifferences in ability to observe will be noted. Some pupils arecareful, others careless, some see little, others much.)Out of the reports of the various students will come much

interesting and profitable discussion. Some teachers will find itwell to prepare a list of questions and to ask the students tofind the answers to these questions which all relate to the factsobserved.

A MAGIC WANDWhat Is Seen:

Water is thrownfrom a glass tube,drawn to a point, insuch a way as to forma spray of very finedrops almost ap-proaching the appear-ance of mist. Thisspray is thrown into

74 SCHOOL SCIENCE AND MATHEMATICS

the air and the water permitted to fall into the sink. When theoperator points a black rod towards the jet of water the dropsare attracted by the rod at one time and at another the streamsuddenly changes from small drops to large ones, or from severalstreams to one single one.

How Used:This experiment may be used in explaining precipitation of

rain from clouds or the condition of colloidal suspension.

Explanation:The fine spray consists of fine drops of water which are all

charged alike electrically. This charge is strong at the upperpart of the spray. Hence the drops repel each other and so donot coalesce. The rod used by the demonstrator is of eboniteand has been charged with.static electricity. When brought nearthe spray the charges on the small drops are neutralized, theyno longer repel each other and so coalesce quickly, forminglarger drops. Near the point where the water leaves the glassnozzle, the drops of water are strongly charged and so stronglyattracted by the ebonite rod sometimes flying several inchesinto the air to reach it.

Construction:Use a glass tube 6 mm. in diameter drawn out to almost

capillary dimensions. Attach this to w^ter supply with a rubbertube about 4 ft. long. Control the force of the water by meansof a pinch cock or else get water pressure by use of a syphonset at just the right height. Charge the ebonized rod by rubbingit with a piece of flannel or a cat^s skin.Some teachers will prefer to ask the pupils to explain what

they have seen, then allow them to discover all additional factsby experiment and finally have them write a report explainingall the phenomena observed.

Other teachers will prefer to have the pupils reason, read andexperiment in finding the answers to a list of questions and thesolutions to problems which they will present.

A COLOR STUNT

What Is Seen:Two colorless solutions are thoroughly mixed in a large beaker

or flask on the demonstration table. There is no immediatecolor change but in a few seconds the solution suddenly changes

DEMONSTRATION EXPERIMENTS 75

to a deep blue color. The operator is able to predict exactlywhen the color change will take place.

Explanation:When iodic acid solution is mixed with sulphurous acid solu-

tion a reduction of the iodic acid, with the liberation of iodinetakes place. If starch is present, blue starch iodide is produced.The reaction requires time to begin but takes place almost in-stantly once it does begin. Concentration and temperature arefactors in determining the time of the reaction after the solu-tions are mixed.

Preparation:1. Dissolve 10 grams of iodic acid in a liter of distilled water.

Place in a gisss stoppered bottle labelled solution A. (Solutionis very permanent.)

2. Bubble sulphur dioxide through 50 c.c. of water in a beak-er till the solution is saturated w^th the gas. Pipette 25 c.c. ofthis into a glass stoppered bottle, dilute to one liter with dis-tilled water and label it solution B. (Solution keeps only a fewweeks.)

3. Pour 480 c.c. of distilled water (20 degrees C.) into a cleanone liter beaker. To this add 70 cc. of solution A. (Iodic acid)and 1 c.c. of clear starch paste (freshly prepared). In anotherbeaker mix 180 c.c. of solution B.(S02) and 320 c.c. distilledwater. Mix each solution separately then pour them both quick-ly into a one liter cylinder and mix with a long clean piece ofglass tubing. In a very definite time (almost exactly one min-ute) after mixing, the color will suddenly change to deep blue.Use a piece of white cardboard as a background and a light toshow color if experiment is performed at night. By varying thedilution, the time of the reaction may be changed. Use metro-nome or pendulum to check time. By experimentation to de-termine the exact time, a very impressive demonstration canbe prepared.

Chemistry students may be asked to determine just how this"stunt^ was done. They may be permitted to examine and ex-periment with the solutions used. Then by study and experi-ment, they may carry the investigation to successful conclusionwhich is indicated when they can duplicate the demonstrationas described above.

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The foregoing experiments have undoubtedlylong been knownto many teachers and have been used by some of them mostadvantageously. Most teachers are not familiar with many ex-periments suitable for demonstrations escept those included inthe usual laboratory manual. In earlier numbers of "SCHOOLSCIENCE AND MATHEMATICS" a few of these less wide-knownexperiments have been described by teachers from time to timebut the attempts to describe the experiments of this kind havebeen sporadic and unsystematic.

In outlining the above experiments the writers have offeredthem as examples of a type of experiment which may be usedas problem demonstrations. Other similar experiments are un-doubtedly known to almost every experienced teacher. Otherswho are willing to contribute worthwhile experiments of anunusual type to these pages are invited to send their contribu-tions to J. 0. Frank, State Teachers College, Oshkosh, Wis-consin. Comments or criticisms of any kind from teachers re-garding experiments of the type described wdll also be gratefullyreceived.

AMATEUR TELESCOPE MAKING AS A PROJECT INHIGH SCHOOL PHYSICS

BY WILLARD GEERReedley Joint Union High School, Reedley, California

The challenge that constructing a telescope makes, the re-ward that it offers to the boys who master the task, and thepatience and care required, make this project one of the mostfascinating of all high school projects. Many have thought uponfirst consideration that the work of telescope constructionwould be too difficult for a high school boy, but the boys ofthis high school have even taken telescope production out ofthe project field and have adopted it as one of their hobbies.

Interest was started when the physics class of the high schooldecided in the winter season of 1931-32 to construct as a classproject a small reflecting telescope of the same type used inthe larger ones at Mt. Wilson. The size was decided upon, a pairof six inch discs were purchased along with the proper grindingpowder, and work was started to hollow out a concavity inone of the discs to produce the mirror. This was done by theprocess of rubbing one disc, the thicker one, over the thinnerone with a mixture of carborundum and water intervening. The