This is a kit of properties, with instruction book for 256 home experiments in Physics, intended for children 10+ years. The French in the title and on the box suggests a bilingual treatment, but that is erroneous. The only French in the box is some alternate instructions on the decal sheet. Apparently the French fraud was only to facilitate sales in Canada. I purchased the kit hoping it would show me how Physics is presented in schools, and present some clever ideas for simple experiments. Indeed, I presume it does present such a view, but the picture is not a pretty one. It is good evidence for discouraging the presentation of Physics in American schools, until it can be done by those more adequately prepared, at the college level.
On page 4, the Celcius temperature scale is mentioned, and reference made to S1 units, which I presume means SI. The corresponding French is not given. On page 34 we have dependant upon; page 58 reveals irregardless; on page 61 we have "effect" where probably "affect" is meant. These are not misprints, but illiteracies testifying to poor editing. There are probably more, but these give the flavor of the style.
Much more serious are the errors in observation and explanation. In Experiment 194 we are spinning a colored disk with alternate blue and yellow segments. "Spin the colored disk by twirling the steel rod between your thumb and forefinger. As the disk begins to spin quickly, the yellow and blue sections begin to merge into a green. The disk reflects both yellow and blue light rays, which the brain combines to form the perception of green. Green is not a primary color, but a combination of yellow and blue." What actually happens (I used an electric drill to rotate the disk) is that the disk appears a pure white. There is no green at all, none!
The writer clearly did not perform the experiment, but gave us what her very great ignorance expected would happen. American education has long had a problem with additive and subtractive color mixing, which saddled innocent students with three-color water color sets that mainly just produced muddy greens. Blue, green and red are additive primaries, while cyan, yellow and magenta are subtractive primaries. Yellow is green + red, while yellow + blue gives white. Not one true concept in this rich field is communicated by this experiment! I wonder what the young victims think when they see white!
Experiment 110 compares the frictional force on moving a book when it is flat on the table, and when it is on its spine. "Notice that you require considerably less force this time (book on spine). Friction is dependant (sic) on the size of the area in contact." Well, I got about the same force in both cases. Once again the author has never done the actual experiment. Every engineer knows that the Coulomb frictional force is proprtional to the normal force and independent of the area of contact. Teachers, appearently, do not know this. This would have been a good place to use F = μN to introduce a little mathematics, but the opportunity is lost.
In Experiment 223, we are freezing water with a salt-ice mixture. The explanation is: "The action of the salt dissolving in the ice water absorbs a substantial amount of heat. That is why the salt-ice mixture freezes the water faster than the ice alone." Absurd. It could easily be observed that the solution of salt has very little effect on the temperature. Some salts do, but not common salt, NaCl. The author is apparently ignorant of the effect of solutes on freezing and boiling points.
In Experiment 224, two ice cubes are made to stick together by pressing them together with the fingers. This is explained as the melting of ice when under pressure, and refreezing when the pressure is removed. The force that can be produced by the fingers is very much smaller than would produce any noticeable effect on the freezing point. This is an imaginary explanation, and teaches nothing correct. One can easily get ice cubes to stick together, but the reason for this is that they are still much below the freezing point when you take them from the ice cube tray, and freeze together because of this. No pressure is necessary.
In Experiments 178 and 179, we are using small drops of water as simple magnifiers. They are described as having concave surfaces, which magnify. Possibly the reason for this is that on the opposite page it is noted that a convex surface reduces the size of a reflected image. The treatment of images and lenses is very inadequate and confusing.
Experiment 190 is the only one dealing with the rainbow, and is pathetically inadequate. The only explanation is that "water droplets in the atmosphere act like prisms to form a natural rainbow." Of course, they do not act at all like prisms. There is no diagram of light paths forming the rainbow, or within a water drop, or is it mentioned that the light is reflected at the back surface of the drop. The rainbow angle is not illustrated, and there is no mention of the outer rainbow, the Alexandrine dark space, or subsidiary rainbows.
The explanation of prisms is also poor. There is no mention that they deviate light rays, and that the amount of deviation depends on the refractive index, so that colors are separated in a spectrum. This is easy to illustrate, and can be clearly demonstrated. No glass prism is included in the kit. The inclusion of a couple of lenses and a prism would be a valuable addition.
Experiment 150 claims that a ticking watch is better heard when on the opposite side of a balloon held to the ear. Whether this is true or not, the explanation that the reason is that the carbon dioxide in the breath used to inflate the balloon conducts sound better than air is absurd. There is not enough carbon dioxide to have any effect. Ticking watches, incidentally, are not easy to find these days.
In Experiment 19, water is boiled in a test tube closed by a cork and tube. It is removed from the heat after most of the water is gone, then the end of the tube is immersed in water. The water, of course, is drawn into the test tube, and will nearly fill it. The text says that in boiling the water, you filled the test tube with hot air which then cooled and created a vacuum. At least it is mentioned that it was the air pressure that pushed the water in, instead of saying it was sucked, a reason used elsewhere. Of course, what you did in boiling the water was to fill the test tube with water vapor, not hot air, and when this condensed to water it produced the vacuum. A careful student might even see the drops of condensed water and wonder how hot air made them by cooling.! It is remarkable that this is a version of a classic experiment for which the correct explanation is always given.
Experiment 20 well illustrtes the principle of the caisson for underwater work. Air is blown into a funnel resting on the bottom of a glass container, with a marble inside the funnel to represent a worker. The instructions say "Perform this experiment in a large glass container so that you will be able to observe the marble from the side." Well, maybe so if you have x-ray vision, since the funnel supplied is of opaque yellow plastic!
Some of the early experiments are designed to show that we are at the bottom of an ocean of air, under considerable pressure, a laudable aim. Curiously, the air pressure is given as "about 1 kilogram per square cm" in spite of the claim to use SI units. In Experiment 25, we read that "air is sucked back", and in Experiment 28 that "the vacuum pulls the water." This is what people thought before Torricelli and others set them straight. Any of this use of "horror vacui", however common in everyday speech, should certainly be avoided while explaining atmospheric pressure, siphons, and such to modern students. No mention is made of the maximum height of a siphon, or the length of a barometric column of water or mercury. In fact, p = ρgh could receive much more attention here.
Archimedes' Principle is enunciated without the smallest explanation why it is true, though this is very easy to do if the increase in pressure with depth is recalled. Worse, it is not mentioned in experiments where it plays an important role, such as measuring the weight of the air in a balloon (Experiment 11). Even on the opposite page we read that "Warm air is less dense than cold air and, hence, tends to rise". There is no hint that this is buoyancy as well, and the "hence" is unsupported. This is an appalling example of "rote learning" that teachers protest in speech, but cling to in practice.
A feature of the kit is the electric motor, Experiment 246. It is not difficult to construct, but one should make sure that the lacquer insulation on the wire is completely removed where elecrical contact is to be made. I used a laboratory DC supply for power, set to current limit at 1.5 A, which runs the motor vigorously. There may be problems when using 2 AA cells, however. Also, contact can be made with the rotor by wires held against the leads, instead of using the wire loops.
It is exciting to see the motor spin, but it will not teach much about electric motors, since its mode of operation is not at all obvious, and is clearly a mystery to the authors of the experiment. They seem to think that it acts like the usual small motors used for this demonstration, with electromagnet rotor and stator, and a commutator for reversing the connections to the rotor coils to provide steady rotation. This motor has no commutator, though the authors keep referring to one and make meaningless comments about it. This motor can be termed an "armature reaction" motor, since it depends on voltages induced in the rotor (armature) by the rotation. Without a battery connected, it is a magneto generating an alternating voltage in phase with the rotation. When a battery is connected, the combination of voltages causes a current that is a maximum or minimum when the rotor is vertical, so that the impulses are alternately larger and smaller. The rotor rotates in the direction of the larger impulses. This is not really a practical motor, since its torque characteristics are poor, and the current supplied to it is not limited by back emf, bu merely by the small resistance of the rotor winding (actually by the internal resistance of the source), wasting most of the energy supplied. A simple motor of this type is sold by Edmund Scientifics that is easier to understand, since its rotor is just a circular coil. These motors are identified by the presence of a permanent magnet and the absence of a commutator.
Motors of this kind have appeared in many experiment kits, since they are so simple to make. In the Science Discovery Kit "Electric Motor/Generator" by Dowling Magnets, the instructions call for removing the insulation on the rotor connecting wires only on one side, so current flows only when the rotor is in the correct position for a kick; it coasts by the 180° position with the current off. Of course this will work, but it must not be easy to do properly. It is exactly what an author knowing about motors might recommend, not trusting a motor without an actual commutator. However, the motor will still work if you scrape all the insulation off. In the Mini-Labs Science Kit "Electromagnetix" it is called the "Mystery Motor" and all the insulation is removed. How it works, however, is allowed to remain a mystery. Another Mini-Lab kit includes a very practical permanent magnet 3-pole armature DC motor that is like those actually used. However, it cannot be explained easily by magnetic attraction and repulsion, and is just as mysterious to the layman as the Mystery Motor.
The usual demonstration motor is of the type invented in the 1830's by S. M. Christie, W. Sturgeon and others. The rotor and stator are salient-pole electromagnets whose poles repel and attract, which is easy for students to grasp. Sturgeon invented the commutator, a rotating switch, to change the connections to the rotor so that continuous rotation was produced. It is easy to explain the operation of the motor on this basis. Of course, there is also armature reaction, but it is not essential to the explanation. There were many attempts to use magnetic forces in practical motors in the 1840's and 1850's, but all failed. The "repulsion" motor became worse when enlarged, either not working or burning up from excess heat. The solution was only found in the 1870's by Pacinotti and Gramme in which the torque generated was accompanied by a back emf accounting for the mechanical energy, and limiting the current accordingly, so that the efficiency rose to 90% and higher, instead of the few percent of the repulsion motor. The repulsion motor was not the ancestor of the modern electric motor, though it does teach valuable lessons.
The most unsatisfactory apparatus furnished was the spring scale, very important for most of the experiments on mechanics and simple machines. It consisted of a helical spring inside the plastic tube used for the syringe and graduated cylinder as well. The spring had a semicircular loop on top with which it was fastened in a cap for one end of the tube. It should have had another at the other end, for attaching the thread for loading the scale. When the thread was tied around the bottom loop of the spring, the loading was asymmetrical and the bottom of the spring was distorted. This was avoided by rotating the lowest loop by 90°, in effect forming a loop similar to the one at the top, and this allowed the spring to be loaded properly.
To calibrate the scale, the 5 g weight was to be suspended from the scale, and a plastic scale attached to the tube with the 5 g mark opposite the bottom of the spring. However, the 5 g weight was insufficient to extend the spring at all. Clearly, the spring furnished was not the one corresponding to the prepared scale. Instead of 25 g for the maximum (with the end of the spring at the bottom of the tube), it was closer to 40 g. The only way to make this work was to calibrate a new scale. The end of the spring was not easy to discern through the semitransparent tube. A search of the springs I had on hand showed that all of them had a much too large constant; finding a spring suitable for a spring scale of 50 g capacity is difficult. Incidentally, they are using grams force here, not the SI newton. I used a Cenco 250 g spring scale I had on hand, and heavier weights than the 1/2, 1, 2, 3 and 5 g plastic weights furnished. No method of supporting the spring scale was mentioned. I used a laboratory support, but one is probably not really required.
The balance was more satisfactory. The only problems were that the nuts furnished for adjusting the beam were too large. The threads for them were 8-32, so it was easy to find nuts that fit. The indicator did not fit firmly in the beam--in fact, it wobbled a lot. This was fixed with a small dab of clear cement. The balance beam is also used for demonstrating levers, and the indicator may interfere somewhat with this. The V-shaped lugs are used as lever pivots. The balance pivot is a short metal wire. This is not nearly as good as a knife edge, since it allows the beam to wobble and has increased friction.
The remaining properties are reasonably satisfactory. The plastic 7 oz/200 ml beaker is quite nice. The eye dropper is functional, and is calibrated to serve as a pipette. The 30 ml graduated cylinder made with the syringe barrel is not needed, since a glass 50 ml graduated cylinder is much more satisfactory. I was not able to check the air pucks, since the balloon furnished broke when first inflated. Possibly a fresh balloon would be satisfactory, if I knew where to find one. A candle is furnished for heating. I would tend to use my alcohol lamp instead, since it is cleaner.
The use of balloons in anything intended for children under 8 years appears to trigger warning labels. This kit is for 10+, but the warnings are still there. The regulations for the use of the batteries (2 AA size) are standard, but quite superfluous here. No one would mind if cells of different ages or manufacturers were used together, and even rechargable cells could be used, though there are dire warnings that they must not be used. The kit contains no glass.
Power Tech Physics Physique, by Tree of Knowledge, distributed by Elenco Electronics, Inc. 150 Carpenter Avenue, Wheeling, Illinois 60090. 250+ Experiments, 64 pp., 8th ed. 2000. No authors listed. Purchased from American Science and Surplus, P.O. Box 1030, Skokie, Illinois 60076.
Composed by J. B. Calvert
Created 25 June 2007
Last revised 10 July 2007