The first device is a rain gauge that automatically records the depth of fall and also empties itself. The idea is submitted by Roger Hayward, whose drawings are a regular feature of this department. The gauge is much more than a measuring instrument; it also illustrates by analogy the functioning of an electronic multivibrator. Such devices are found in television sets, space vehicles and digital computers.

The multivibrator of Hayward's gauge is a fluid oscillator-a miniature seesaw powered by water [see illustration on the left]. Essentially the gauge is an open-end trough divided by a thin crosswise partition in the middle. The trough pivots on a pair of supporting legs under the partition.

The water flows into the structure as a narrow stream from a position directly above the middle of the partition when the trough is horizontal. The trough never comes to rest in the horizontal position, however, because it is top-heavy and hence bi-stable. When the apparatus is at rest, one end of the trough is always down and the other up. Accordingly the stream plays against the side of the partition that faces the upper end of the trough.

As this end fills, the accumulating weight ultimately flips the seesaw, discharging the water through the open end of the trough. The stream now plays against the other side of the partition and so initiates the second half of the cycle. Electronic multivibrators, which are sometimes called free-running "flip-flops," operate in the same manner. Two capacitors, counterparts of the divided trough, alternately charge and discharge through a circuit arrangement, which includes transistors or vacuum tubes and constitutes the remainder of the seesaw.

The fluid oscillator can be constructed so that each flip of the trough discharges a predetermined volume of water. When the volume is known, the depth to which a container of known diameter would be filled is easy to calculate. The container that catches rain in Hayward's gauge is a sheet-metal box 15 inches square constructed with a sloping bottom and a drain in one corner [see right]. A vessel of this size holds 225 cubic inches of water per inch of depth. The trough of Hayward's fluid oscillator is designed to dump 2.25 cubic inches of water per flip. One hundred flips are therefore equivalent to one inch of rain.

In order to record and display the depth of rainfall Hayward constructed a ratchet motor actuated by a pair of pawls linked to one of the pivot arms of the seesaw. The ratchet wheel advances one tooth per flip-flop and makes one full revolution per inch of rain. A dial on the face of the ratchet wheel is calibrated to indicate .1 inch of rain per division and .01 inch per subdivision. A train of reduction gears that mesh with the ratchet wheel drives the pointer of a second dial for indicating total rainfall to a depth of 16 inches.

Experimenters who do not have the gears can substitute an electrically operated readout device. A set of relay contacts could be fastened to the fluid oscillator for closing a circuit each time the device flips. Electric pulses so generated would actuate an electromechanical counter of the ratchet type. Such counters are available inexpensively on the surplus market. The counter could be installed indoors.

The accuracy of the gauge will be no better than the workmanship that goes into its construction. On the other hand, it is easy to adjust the device after it is assembled. Make the seesaw so that it will flip about 5 percent faster than the desired rate. Then attach an adjustable weight to the top of the partition. As the weight is made heavier increasing amounts of water will be required to flip the mechanism. Find the proper amount of weight by experiment. Insert a piece of screening in the vessel that catches the rain, the screen prevents drops from splashing out of the vessel before they are measured.

James Sharpsteen of Glendora, Calif., submits another version of the fluid flip-flop, this one operated by hot air. It consists of a pair of smoke channels in the form of a V, which is open at the bottom for receiving smoke from a lighted candle below, and two ports that open into the channels on opposite sides at the apex of the V [see bottom illustration at right]. Puffs of air can be blown through the side ports by mouth by means of short lengths of flexible tubing. The smoke channels are made of plywood and are closed at the front of the apparatus by a sheet of glass.

When the candle is lighted, smoke immediately enters just one of the output channels. If a gentle puff of air is now sent through the port on that side, the smoke will switch to the other channel and stay there. Hence the device is bi-stable. A puff through the other port will switch the smoke back to the first port again. The switching action is surprisingly sensitive. It will occur even if the experimenter puffs gently when he is holding the blowing tube several inches from his mouth.

"This quite simple apparatus," writes Sharpsteen, "is an example of an important class of control devices. I have in mind particularly those devices used for amplifying signals, performing computations and making logical decisions [see "Fluid Control Devices," by Stanley W. Angrist; SCIENTIFIC AMERICAN, December, 1964]. I constructed it to help satisfy my own curiosity about such devices."

Having set up the apparatus and switched the smoke back and forth a few times, eliminate the left-hand control port (the wedge is removable) and note that the flow now attaches itself automatically to the wall on the right side, except when a "signal," in the form of a puff, is applied to the remaining control tube. The device is now an inverting amplifier or, in the terminology of computer specialists, a "not" element, because an output flow occurs in the right channel only when there is not an input signal.

The mono-stable characteristic of the not element is due to the difference in the amount of resistance presented to the flow of air by the openings at the apex of the V. The difference is not large; therefore the action is not always reliable. Adding to the length of the control tube helps, but the best solution is to cut away the wall on the left side.

Now imagine that there are two or more tubes on the right side. Suppose there are three: A, B and C. A continuous stream of air in any one would switch the flow to the left side. That side would then perform as an "or" function, meaning that it would respond to a signal in one or another of the available channels. In this case an output would be produced whether the signal entered tube A or B or C. Operating the fluid flip-flop to discover various methods of generating logical functions can be instructive as well as fun.

The device is also instructive from the viewpoint of fluid dynamics. The reason is that the flow is clearly visible, both in the smoke itself and in the patterns of soot that are deposited on the glass. Records of the soot patterns can be made by removing the glass carefully and using it as a photographic negative for making contact prints.

"In comparing the performance of the apparatus with fluid amplifiers that operate on compressed air," Sharpsteen writes, "one must realize that the candle generates only feeble energy in terms of the resulting convection current in the air and that the channels are extremely large with respect to those in conventional fluid amplifiers. The action is therefore somewhat sluggish. Moreover, because the motive power consists of convection currents, the jet can become biased to one side if that side grows significantly warmer than the other. These limitations are balanced somewhat by the fact that laminar flow and leisurely switching speed enable the experimenter to see what is happening. This is an advantage one does not have in the case of conventional fluid amplifiers."

As for the details of construction half-inch plywood can be used if the nozzle area immediately above the candle flame is protected by sheet metal, such as aluminum. The dimensions specified in the drawing are fairly important but need not be followed exactly. Making certain parts adjustable, such as the smoke-splitter that divides the channels of the V, might be an interesting variation on the design.

While Sharpsteen was having fun in the forefront of modern technology, Frank Cousins of Sussex, England, turned to the 17th century for diversion and came up with a portable sundial invented in 1630 by a Jesuit priest named Francois de Saint-Rigaud. The sundial can be duplicated in a short time by anyone able to manipulate a compass and straightedge.

Cousins' version of the dial is made of cardboard, a bit of thread, a bead and a small weight [see illustration at left]. The device is held by hand at an angle such that a beam of sunlight falls on a "sun line" drawn on the card. The hour of the day is then indicated on the dial by the position of the bead. The position at which the plumb line is attached to the card must be shifted during the year to compensate for the changing angle of the sun. The proper point of attachment is designated by a scale calibrated in months.

To duplicate the dial cut a rectangle about four inches wide and six inches long from a sheet of durable cardboard. With a pencil and straightedge lightly draw the sun line parallel to and about a quarter of an inch from one end of the card [see top illustration at left]. A second parallel line is drawn about two inches from the opposite end. The left end of this lower line is marked "Noon point." Through the middle of these parallel lines draw still another line at right angles to divide the card into equal parts. (Only a segment of this line is inked when you complete the sundial.)

With the intersection of the lower line and the dividing line as a center, draw a semicircle of any convenient radius below the lower line. Divide the arc into 12 equal parts. From the noon point draw a line that inclines upward to the right at an angle equal to the latitude of the geographical location at which the dial will be used. Call this the "latitude line." It will intersect the perpendicular that divides the card. At the point of intersection draw a line at right angles to the latitude line, as though crossing a T. This is the "date line." It will be divided into intervals corresponding to months and will be used for locating the position of the plumb line.

Using a protractor, extend lines from the noon point that make an angle of 23 degrees 30 minutes with the latitude line. These lines will intersect the date line both above and below the latitude line. The lower edge of the date line must now be divided into six monthly intervals to reflect the changes in the sun's declination between solstices. Proceed from left to right. Using the protractor and straightedge, determine the point at which a line would intersect the date line if it were drawn at an angle of 23 degrees between the noon point and the latitude line. It is not necessary to draw the line. Just mark the point of intersection below the date line. Now determine the point of intersection if the line were to make an angle of 17 degrees 30 minutes. Mark the point of intersection Th space between these points is designated January. Similarly, determine the point of intersection of a line drawn at 8 degrees. Mark the point on the date line and label the interval February. The line for March 31 lies on the lower side of the latitude line at 4 degrees, April 30 at 14 degrees 30 minutes, May 31 at 22 degrees and June 30 at 23 degrees 15 minutes.

Proceeding from right to left, lay off the remaining six months on the upper side of the date line. July 1 appears at the lower right end of the date line; July 31, to the left of this point at 18 degrees 15 minutes; August 31, at 8 degrees 30 minutes; September 30, at 2 degrees 30 minutes, to the left of the latitude line; October 31, also left of the latitude line, at 14 degrees; November 30, at 21 degrees 30 minutes, and December 31, at 23 degrees.

With the upper end of the date line as a center and with a radius equal to the distance between this point and the noon point, draw an arc intersecting the line that extends to the right of the noon point. With the same radius draw a similar arc, using the lower end of the date line as a center. These are the hour arcs. Finally, rule a set of parallel lines through the 12 intervals of the first arc you made. These lines are drawn parallel to the line that divides the card in half. Ink in the pencil lines and label the dial as shown in Figure 4.

The plumb line consists of the thread and weight plus a small bead that makes a snug but sliding fit with the thread. Attach the weight to one end of the thread. String the bead. With a razor blade cut a slit through the date line. Insert the thread through the slit and cement the free end to the back of the card at a point near the bottom at the center. To the end of the sun line, at the right, attach a rectangle of cardboard about the size of a postage stamp by means of a linen hinge. This is the folding sight. Make a vertical slit in the rectangle about a sixteenth of an inch wide. (You may prefer to do this before attaching the sight.)

To read the time first slide the thread in the slit of the date line to the appropriate day of the month. The position will of course be an approximation. Pull the thread snugly over the latitude line and set the sliding bead directly over the noon point. Open the sight, hold the card vertically and turn it so that a shaft of sunlight streams through the slit and falls on most or all of the sun line. Release the plumb bob, and the position assumed by the bead marks the hour.

"This dial," writes Cousins, "is satisfactory for all locations in the Northern Hemisphere. In the Southern Hemisphere the order of the intervals on the date line must be reversed. The dial is no toy but a small geometric and astronomical instrument of remarkable accuracy. It gives much satisfaction and should be boldly inscribed on the back with the ancient Chinese proverb: 'An inch of time on the sundial's face is worth a yard of jade.'"

Daniel Gordon, a retired New York pharmacist, admires jewels of a different kind. He makes his own by crystallizing various harmless chemicals; as they grow he watches enlarged images of them on a projection screen. Gordon first makes up a thin rectangular cell from square cover glasses of the kind used for projecting 35-millimeter color slides [see Figure 6]. A piece of cardboard, such as a business card, is sandwiched between a pair of the glasses and clamped by a clothespin of the spring type. Three edges of the glasses are sealed with a thin layer of epoxy cement. When the cement has hardened, the clamp and card are removed.

A small quantity of a chemical such as thymol, menthol, resorcinol or salol is now placed in a teaspoon and melted by heating the spoon slowly over a gas flame. When the substance melts, approximately one milliliter is taken up by a medicine dropper and put into the cell. The cell should be filled to within about an eighth of an inch of the top. The opening is wiped with a soft cloth to remove the excess chemical and is sealed with epoxy. After the cement has hardened, the completed cell is ready for use. It can be mounted in an aluminum slide holder.

To observe the growth of crystals warm the slide to melt the chemical. Gordon does this by placing the slide flat on a sheet of asbestos paper that he then heats slowly on his kitchen stove. When the chemical has melted, the hot slide is placed in a conventional 35-mm. projector and focused on the screen. Wear a glove when handling the hot slide. Within a minute or so, depending on the temperature of the slide, the image of one or more minute crystals will appear on the screen. Within seconds these centers will grow explosively into lacy or dendritic patterns that fill most of the screen. Simultaneously crystals of differing patterns will form in the space above the fluid. These grow from the vaporized chemical.

Each chemical crystallizes in a pattern that is characteristic of the substance, but no two patterns of the same chemical are ever identical. The crystals of some substances, such as resorcinol, are optically active: they tend to cause light waves to vibrate in only one plane. When such crystals are examined between two sheets of high-extinction Polaroid-set at right angles to each other so that the pair does not transmit light-crystals appear in all the colors of the rainbow. The color effect can be projected onto the screen by placing a slide of Polaroid behind the glass cell and a second slide in front of the projection lens. The front slide is then rotated until the colors appear.

Gordon is now experimenting with additional chemicals such as acetamide, calcium nitrate, magnesium nitrate, oxalic acid, naphthalene and potassium aluminum sulfate. He invites others to join in the hunt for additional substances that melt and recrystallize at moderate temperatures. According to Gordon, interesting variations can be induced by making wedge-shaped cells. The business card is inserted at only I one edge of the glass cell. When the assembly is clamped, the opposite edges of the glass are in contact. The wedge-shaped cavity so formed restricts the crystallization increasingly toward the bottom of the cell and results in a continuous change in the pattern from top to bottom.

In another variation of the technique Gordon binds a short length of fine resistance wire across the bottom of some slides. Iron wire removed from window screening can be used, as can a strand of equally fine Nichrome wire. The ends of the wire are connected in series with a rheostat of about 30 ohms and to the low-voltage terminals of a 12-volt transformer when the slide is in position in the projector. When power is applied, the wire heats and melts the chemical. By adjusting the rheostat the experimenter can control the rate of cooling to alter the size and shape of the resulting crystals and can enjoy a continuously varying display without disturbing the apparatus.

Bibliography

UNESCO SOURCE BOOK FOR SCIENCE TEACHING. United Nations Educational, Scientific and Cultural Organization, 1962.

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