This Message Was Brought To You By the Letter “E”
Pilots rightfully have something of a vested interest in that benchmark of assessing their visual acuity: namely, the eye chart. Why is it that the letter E is usually the topmost letter?
- This is a trick question; it actually isn’t.
- Because it has the greatest number of lines for the space it occupies, it is actually the hardest of all upper case letters to differentiate. (The fact that it gets top billing is simply its “handicap”.)
- The reason is that Dr. Hermann Snellen found that the letter “E” shares several qualities with the greatest number of other letters (e.g., B, F, P, etc.)
- The letter E appears atop most eye charts only because it is the most frequently used letter of the English alphabet.
- It’s because it most closely mirrors the attributes for testing vision (a series of parallel lines) that had been used by the predecessor of the famous Dutch professor of ophthalmology, Dr. Snellen, also the originator of what we now see (with varying degrees of success) as the modern eye chart.
Making the grade
What is the meaning of those cryptic numbers used to identify engine oil?
- the range of freezing temperatures
- a range of viscosities
- its range of operating temperatures in degrees C, divided by 10
- the range of densities in grams per deciliter
On Thin Ice
A pilot of an airplane without floats would be glad to have a frozen lake to land on, if that engine (or engines) ever quit while out of reach of land. But just as hot air rises, then why does ice form first on the surface of lakes, ponds, rivers, and even oceans?
- It is because evaporative cooling releases about 540 times the heat of a one Centigrade degree warmer parcel of water, per gram (despite radiative heating from the sun).
- Due to the viscosity of water, mixing doesn’t occur to any significant degree, and the latent heat of cooling (the same excuse as in the previous choice) functions to reduce the surface temperature, first.
- Water actually contracts as it cools, becoming more dense (not less dense, as you would expect from having seen everything from icebergs to the ice cubes in your drink). However, that is only up until the point where it cools to four degrees Centigrade (about 39 degrees Fahrenheit). After that, it expands, becoming less dense, and rises to the surface. When it freezes, its density drops even more significantly (further assuring its higher status).
- If water were absolutely pure, it would form at the bottom first. However, the vertical profile of particulates causes the freezing point to be lowered. This is greatest at the bottom, and least at the surface. This is true for lakes, rivers, and even oceans.
The Answers…
This Message Was Brought To You By the Letter “E”
Answer: The answer is E (appropriately enough), because the letter E (as upper case), with its three horizontal lines and white spaces between them, forces an observer to distinguish between white and black, and is perfect for measuring how small an image a person’s eye can perceive and still identify the characteristics of that image. (Some letters are actually more difficult than others, such as the relatively easy letter L, with its open spaces.) It also most closely resembled the formulas for visual acuity that were based on parallel lines, which were developed by the Director of the Netherlands Hospital for Eye patients, Dr. Frans Donders, who was, in the mid-nineteenth century, the world’s leading authority on optics. He based his fairly complex geometric algorithms quantifying how the eye resolves and differentiates between what it sees on this letter. His successor Dr. Hermann Snellen actually first used the letter “A” atop his eye chart in 1862, but soon switched to the letter “E”. Actually there are only a limited number of letters (perhaps ten) on Snellen charts, in order to minimize the range of difficulty, and the letter E in fact does not always get top billing. The well known Snellen quotients relate to a person’s ability to identify letters of a certain size at a specified distance. They give no information as to whether or not any meaning is obtained from them, how much effort is needed to see them clearly, or whether or not both eyes are used, as opposed to each eye individually. In the Snellen fraction 20/20, the first number represents the test distance (20 feet). The second number represents the distance that the average eye can see the letters on a certain line of the eye chart. So, 20/20 means that the eye(s) being tested can read a certain size letter when it is 20 feet away, as well as a having what is considered normal vision. (The standard definition of normal visual acuity, or 20/20 vision, is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc, or 1/60 of a degree. In most people who have normal abilities for spatial resolution, their limit is derived from the fact that each degree of whatever they see is projected across 288 micrometers of their retinas by their eye’s lenses. In most people, within this 288 micrometers dimension, there are 120 color sensing or central vision cone cells. Thus, if more than 120 alternating white and black lines are crowded side-by-side in a single degree of viewing space, they will appear as a single gray mass to the human eye. Some folks like Ted Williams or Chuck Yeager, in addition to their eyes having had nearly optimal optics, were just blessed with more.) By the way, in case you’re wondering, getting the top line right at your next flight physical isn’t much to brag about. (That’s 20/200 vision.) But if you get the bottom line, well, that is. That represents 20-10 vision. (And you don’t have to be Chuck Yeager, either. A little-known standard dictates that credit be given if you succeed in identifying only a majority of the letters on a given line.)
Making the grade
Answer: There are several important properties of a motor oil, including viscosity, flash point, pour point, as well as the percentages of sulfated ash and zinc. One of the most important is viscosity. Scientifically speaking, it is the property of an oil to develop and maintain a certain amount of shearing stress dependent on flow, and then to offer continued resistance to flow. Thinner oils with too low a viscosity can shear and lose film strength at high temperatures. Oils with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm. The weights assigned to oils are numbers assigned by the Society of Automotive Engineers, and which correspond to “real” viscosity, as measured by several accepted techniques. The numbers indicate viscosities; higher numbers mean greater viscosity. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50, or 60 by the S.A.E. The W means the oil meets specifications for viscosity at 0 degrees Fahrenheit and is therefore suitable for winter use. Multi-grade oils have a dash between two numbers, one of which is a low value with the other being an upper range. The range of viscosities isn’t some schizophrenic property of the oil, but two different viscosities at two extremes of temperature (typically a low viscosity during winter and a high viscosity during the summer). With multi-viscosity oils, polymers are added to a light base such as 5W, 10W, or 20W, which prevent it from thinning as much as it warms up. When cold, the polymers are coiled up and allow the oil to flow as their low numbers would indicate. As the oil warms up, the polymers unwind into long chains, preventing the oil from thinning as much as it normally would. Thus, a 20W-50 oil is like a 20 weight oil that will not thin more than a 50 weight oil would, when hot. (Yes, it’s choice B.)
On Thin Ice
Answer: It sounds nutty, but water really does achieve its greatest density at 4°C. As water cools, it sinks to the bottom of the barrel (or pond, lake, etc.) -up until the point when the entire body of water is uniformly cooled to four degrees, that is. Once that happens, water that cools further becomes less dense, and it starts rising to the surface. As ice crystals form, they expand further by roughly 10%, to a much larger volume. Once the surface freezes over, water beneath will take a much longer time to cool enough to freeze as well, because the surface layer now acts as insulation for the water beneath. Incidentally, this cooling and then freezing of water doesn’t occur smoothly and without a price. When water molecules slow down enough to change from vapor to liquid, or further, to ice, the kinetic energy of their movement changes into (and releases) another form of energy: heat. At the freezing point, significantly more heat is lost per gram of water (dozens of times more than for each Centigrade degree change in temperature). This is called the latent heat of fusion. It is 80 calories per gram (which is about one seventh the latent heat of cooling from the gaseous to the liquid phase, of about 540 calories/gram.) The right answer is choice C.
