I think I first encountered the term “thixotrope” in conjunction with epoxy and the additives you can mix it with. I remember reading about fumed silica, and was amazed that such a product could be created and sold commercially that was so small in particle size. I’ve worked with fumed silica, and indeed it is an amazing substance.
I suspect that other people might conceptualize the property of thixotropy much like I did, and imagine that it’s a property of a material. But if you look at the definition of thixotropy, at least that given by Wikipedia, you notice that it’s a property of “certain gels or fluids.” What got me thinking about this is that a material can become more or less thixotropic depending on its physical state.
I remember sitting at a Mexican restaurant in LA with some of my cousins back in the summer of 1984 and one of my cousins was pouring a carafe of frozen margarita mixture into a glass. He tipped the carafe higher and higher, but the icy mixture still wouldn’t flow. All of a sudden the mixture started flowing catastrophically, at least as far as the glass and table was concerned… One of my other cousins remarked something to the effect that “he wondered if that was going to happen.”
If I understand the concept of thixotropy correctly, then I believe that water is a sort of “auto-thixotrope” in that it’s a material that can become thixotropic depending on its physical state. A fine grained ice slush, like that in a Slurpee or a margarita, exhibits thixotropy. Neither ice nor water by itself is thixotropic, yet a mixture of the two is.
Perhaps I should qualify the last concept, as when I think about it, I suppose that a block of ice would behave differently physically than the same mass of ice broken up into cubes. Perhaps snow is thixotropic, as I think an avalanche might qualify as an example. So now I have to wonder if particle size, particle shape and temperature are factors as well. With water, or more accurately snow or ice, you have further complications conceptualizing this, as you have the molecular lattice structure on the microscopic scale, as well as the “particle” size and shape on the macroscopic scale. The drink in your hand behaves differently as a material depending on the size and shape of the ice “particles” inside. A Slurppe pours differently than Kool-Aid with ice cubes.
So it makes more sense to me how the definition of “thixotrope” is constructed broadly, to encompass “certain gels or fluids” and not strictly as a property of a material. There is a lot more going on than one simple physical property.
I remember a physics class in college where I was introduced to the fact that there was an entire branch of materials science known as “rheology.” At the time I was amazed that an entire branch of science could be devoted to such an esoteric thing as fluid flow. Now it makes more sense, as I can begin see how complex it really is!
Most consumer grade laser pointers come with a momentary switch. Some people might want to have a conventional on/off switch instead, say for photography. Here’s a quick and easy modification that requires no switch replacement or re-wiring.
Start by wrapping tape around the barrel of the pointer right next to the button switch. Build up the tape to a height of about one millimeter. Don’t wrap with a lot of tension, or you may experience the dreaded “tape creep.” Wrap two bands which will act as retainers to keep the modified switch from sliding up or down the barrel.
Find a zip tie that’s roughly the same width as the momentary switch button. The zip tie I’m using is about 5mm wide. If you use a wider zip tie, your tape retaining bands will have to be moved aside to the same width as the zip tie. Encircle the zip tie over the barrel in between the tape retaining bands. Contract the zip tie until you come near the button, positioning the zip tie locking lug over the button. As the zip tie contracts, it will form a teardrop shape, with the locking lug forming the pointed end. Carefully contract the ratchets of the zip tie until the lug almost touches the button but does not turn it on. This is the new “off” position, and should look like this:
Rotating the tie will remove the slack under the locking lug and depress the momentary switch. This is the new “on” position, and looks like this:
Cut off the excess “tail” of the zip tie, and you now have a reasonably elegant on/off switch.
I was able to use my modified laser to capture this image, which is a green glass sculpture in Seward Park here in Seattle:
Several years ago, I was a juror on a trial in which Goeffrey Loftus was an expert witness for the defense. He testified regarding the fallible nature of eyewitness testimony. If his name sounds familiar, it’s because he’s the ex-husband of noted memory researcher Elizabeth Loftus. In an effort to discredit Loftus the prosecution asked whether Loftus had also investigated the moon illusion, which of course he had. It was an obvious appeal to the lowest common denominator intellectually, as there are always going to be those who have no idea what the moon illusion is, therefore the study of it must be “loony.”
I’ve read a number of essays regarding the moon illusion, some written prosaically and some highly technical. In all the work I’ve read on the subject, I’ve yet to come across what I believe is a rather simple possible explanation, and one whose fundamental principles were understood several hundred years ago!
At this point I need to make an admission. I have failed to do the serious bibliographic work required to get an essay like this taken seriously. I don’t have a degree in psychology or art history so some might dismiss my musings due to lack of credentials. I accept that, yet I’m convinced that my suggestion is at least plausible. Please take this essay for what it is, a suggestion, a preliminary sketch of an idea, not a rigorous argument.
For those not familiar with the moon illusion, it’s the psychological phenomenon whereby a full moon on the horizon seems unusually large; larger than when it’s high in the sky. Astronomers are quick to point out that it’s not an astronomical phenomenon, and defer to psychologists and those specialized in human optical and spatial perception.
For many people “perspective” in art means spatial perspective, i.e. how is three dimensional space depicted? But there is also “aerial perspective” which is (roughly) what effect the atmosphere has on perception of objects at a distance.
Let’s take a look at two paintings from the Renaissance, which will hopefully demonstrate what I’m talking about. The first is Giovanni Bellini’s Pieta’. Obviously the figure of Christ is the focus of the painting, but carefully examine the hillside behind Mary. It doesn’t seem quite “right,” does it? It almost feels composited, as if George Lucas had created it.
Now compare Bellini’s painting to Da Vinci’s Virgin of the Rocks. The rocks in the far distance are far more realistic, and part of the reason why is that Da Vinci’s work was more attentive to aerial perspective than Bellini’s.
When objects are seen at great distances in our atmosphere, they are seen through large masses of air. This tends to do several things. It reduces the contrast of the object as compared with objects at close range, and it reduces detail.
A weird counter-example is how astronauts walking on the moon have reported underestimating the length to big boulders seen at a distance. Da Vinci understood this hundreds of years ago, and astronauts of the 20th century discovered it for themselves: The mass of air between one and a distant object affects one’s spatial perception of the object’s distance.
So how does this factor into the moon illusion? When the moon is at the horizon, several factors are in effect. First off one is looking through a greater mass of air than when the moon is high in the sky. Just like the distant rocks in Da Vinci’s painting, the moon has less detail than when it’s high in the sky. Because the full moon rises not long after the sun sets, the moon also has less contrast against the still-illuminated sky in which it rises.
In my opinion, these two characteristics are sufficient to trigger the brain’s natural perception that the moon is at a great distance. From there our brains naturally adjudge the object to have great absolute size, thus the perception that the moon is larger at the horizon.
In my opinion, the moon illusion is nothing more than a rather unique example of the brain’s natural reaction to aerial perspective.
Everyone knows that people, including themselves, can be mistaken. Our egos get bruised when we are confronted with our own mistakes of memory, and we often secretly believe that our memory is better than that of others. In day to day life, being mistaken is usually no big deal, but when it comes to the criminal justice system, it becomes a vitally important subject. Just how accurate is eyewitness testimony?
Only quite recently in the history of psychology has this phenomenon been studied in a careful and scientific way. One of the most well known researchers on the subject is Elizabeth Loftus. In the course of Loftus’ research, she went as far as to introduce false memories into test subjects; fortunately the false memories were of a benign nature!
Skeptics often point to the fallibility of eyewitness testimony, as much of what constitutes “fringe science” is based on eyewitness accounts of transient phenomena.
I believe that it’s important for anyone seeking to develop their own critical thinking skills to learn about the phenomenon of false memory. While there is now a great deal of information available on the Internet and in various books and magazines, I should like to offer a simple test that anyone can do that should really drive the point home.
As an aside, I’d like to share how I discovered this. It was quite by accident. Back in the mid to late 1970’s VCRs were expensive and uncommon. Hollywood resisted distributing movies on tape for years. In many cases, the only way you could see a film more than once was either if it was shown on television, or else if it became a “cult” movie and shown at midnight in a theater.
I remember seeing the movie Tommy when it was originally released. At the time, I was really only familiar with the song Pinball Wizard, which was a big radio hit. As time went on, I became a huge fan of The Who, and bought as many of their albums as I could. Several years after its original release, Tommy was re-released as a midnight movie. I waited anxiously to see it, and would re-play certain scenes in my mind’s eye again and again. I remember being particularly taken by the hang-gliding scene during the song Sensation.
When I was watching the film the second time, the song Sensation came on, and I was shocked to realize that my memory of the scene was quite different from what was actually on the film! At the time I was quite shaken by this experience, and had no resources available to understand how or why this had happened to me. Back in the 1970’s there was no World Wide Web, and no popular literature on the subject, at least that I knew of. I don’t know if the term “false memory” was even in use back then.
As I said earlier, people often secretly believe that their memory of events is superior to that of other people. It’s easy to see when other people make mistakes of memory, but it’s much tougher to accept that one’s own memory is not infallible. But it’s my opinion that realizing one’s one infallibility is a much better learning tool than simply seeing the phenomenon in others. The trick is to do it in a relatively painless way.
Now that most of us have either a DVD player, Internet access or both, it’s easy to perform the following simple self-test. Just pick your favorite film or TV show episode, hopefully one you have watched multiple times. Pick a scene that you feel you know well. Now, write down or at least make mental notes of certain details of a technical nature. What exactly was the character wearing? What position was the camera in? Did the camera move, or was it static? Did character X move right or left during the scene? Where was character X positioned in relation to character Y? While on the bridge of the Enterprise, did Kirk’s evil duplicate in the episode “The Enemy Within” have scratches on the right or left side of his face?
When I saw the movie Tommy the second time, the mental picture I had of Tommy hang gliding differed markedly from what was actually on the film in both the distance and angle from the camera. I’ll bet your memory is good, but I’ll also bet it’s not perfect, and that’s the point. If you’re like me, you will think you know your TV or movie scene perfectly, but you will not.
Advocates of various “fringe sciences” often buttress their claims by appeals to the good moral character or high-status job of the witness. The reality is that human perception and memory is simply not a function of one’s character or job description. My hope is that my simple thought experiment should demonstrate that “if it can happen to me, it can happen to anyone.”
In light of the recent kerfuffle regarding airport strip searches, I was quite surprised to discover this little gem from the dark ages of 1974. Written by Paul Wahl, it appeared in the May 1974 issue of Science and Mechanics magazine, on pages 77 and 93.
Higher resolution scans of both pages can be found here and here.
Nude air travel has been proposed as probably the best way to eliminate the skyjacking menace. Concealing a weapon would be difficult and certainly uncomfortable! This idea isn’t feasible – travelers aren’t ready for it yet- but The Stripper, Tetron’s (sic) new passenger screening device (PSD) might be the next best way of discouraging air pirates.
 Is this the ultimate weapon against skyjackers?  Page 93 When a passenger carrying a weapon-size metal object, ferrous or non-ferrous, walks through the Stripper’s free form arch which contains the search coils, a red light flashes and a beeper sounds off – gotcha!
Don’t let this system’s name and the naked lady in the picture fool you. The suspect’s clothing isn’t dematerialized – no, science hasn’t advanced that far as yet. The name “Stripper” is intended to suggest that this PSD is as effective as a strip search in finding concealed weapons.
Here’s how this detector works: It operates on the principle of measuring the decay time of a magnetic field. A low-value magnetic field is established by a transmitter coil. A receiver unit samples this field with a similar coil. Then the transmitter is turned off, causing the field to decay rapidly. The rate of decay is measured by the receiver and a normal value is established for the rate. When extra metal is introduced into the field, this rate of decay changes, upsets the balance of the normal value, and the receiver produces an alarm condition. Removing the metal from the field produces the same effect in the opposite direction, providing a second chance for detection.
A totally solid state device, the Tectron PSD employs computer-like logic cirsuits (sic) to control the entire sequence of operation. The weapon sensitivity adjustment can be set so that the Stripper will detect a small pocket handgun such as a .25 automatic, but will reject a bunch of keys or other metal objects of lesser mass.
This PSD is a takeoff from Tectron’s Tramp Metal Detector used extensively in ore mines and in the aggregate industry for the protection of crusher equipment.
Currently, the Stripper is under consideration by Canada’s Department of Transport. Manufacturer of this anti-skyjacking device is Tectron Engineering, Santa Ana, California.
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