MANY AMATEURS have built a helium-neon laser. Relatively few have succeeded in using the laser to make a good hologram: the type of photograph in which the coherent light of the laser is used to create an interference pattern that, when it is illuminated with coherent light, yields a three-dimensional image of an object. Most of the failures can be traced to vibrations that produce minute changes in the distances between the holographic optical system, particularly the distance between the object to be photographed and the film holder.

In the making of a hologram the coherent light of a laser is projected along two paths to the sheet of film. The ray traveling along one path constitute the reference beam; they fall on a flat mirror from which they are reflected to the film. The rays traveling along the second path constitute the object beam; they fall or the object to be photographed, from which they too are reflected to the film. The rays of the reference beam are virtually equal in length. The rays of the object beam are reflected by the various surfaces of the object and are therefore of different lengths. As a result the light waves in the two beams are out of step and interfere with one another where they combine at the surface of the film At some points on the film the crests of waves from the two beams coincide. The light is brightest at these points, and the film gets maximum exposure. At other points the crest of a wave in one beam coincides with the trough of a wave in the other beam. The light is dimmest at these points and the film gets minimum exposure. The resulting photographic pattern is essentially a record of the local differences in the length of the rays of the reference beam and the object beam. If the differences in path length so recorded arise solely from the shape of the object, the hologram yields a remarkably realistic image of the object. In practice, however, some portion of the difference invariably arises from the vibration of the apparatus. The quality of the hologram is degraded in proportion to the intensity of the vibration.

Professional workers who make holograms resort to elaborate schemes for isolating their apparatus from vibrations, such as mounting the parts on a multi-ton slab of granite, on a base that floats on air or on a platform equipped with a servo system that cancels the vibrations by means of feedback. People who do not have access to such apparatus can still make reasonably good holograms by means of a simple technique recently developed by LeRoy D. Dickson of the IBM General Systems Division in Rochester, Minn.

"In 1967," writes Dickson, "I found that good three-dimensional holograms could be made by minimizing vibrations that alter the distance between the object and the mirror. Vibrations between the film and the laser, and between the laser and the object, have substantially less effect on the quality of the hologram, although they should be minimized. Essentially what I did was tie the two reflecting elements of the system together with a steel rod so that they moved as a unit. I attached the mirror to one end of the rod by an apparatus clamp and cemented the object to be photographed to a base of sheet metal that was in turn cemented to the rod a few inches beyond the mirror. The remaining portion of the rod was clamped to a pair of apparatus stands for support [see illustration, left]. This improvised optical bench was placed on a solid base that also supported the laser and film holder as separate units. A 45-power microscope objective lens was inserted in the beam of the one-milliwatt helium-neon laser to spread the narrow beam of the laser into diverging rays. The rays were directed toward the optical bench to simultaneously illuminate both the mirror and the object. The resulting holograms were sharp, bright and displayed good parallax.

"Two students at the Mayo High School in Rochester, Brandon Dallman and Tom Smyrk, experimented with the system and achieved excellent results on their first try. For objects they used miniature models of buildings and automobiles and mounted them as close to the mirror as possible. Indeed, one object, a Maltese cross, was mounted directly on the mirror. The one-milliwatt helium-neon laser, the microscope lens, the film holder and the rod assembly were placed on a pool table that rested on the concrete floor of a basement. I recommended the use of four-by-five-inch Kodak spectroscopic plates (Type 649-F).

"Light reflected to the film by the mirror should be approximately four times brighter than the light reflected to the film by the object. To achieve this ratio I use rough-surfaced objects of comparatively high reflectivity, such as unpainted models. The relative brightness of light that reaches the film can be judged by inserting a piece of white cardboard in the film holder and, with the room lights off, alternately blocking the rays from the mirror and from the object with a piece of black cardboard. Light reflected by the mirror can be reduced by moving the mirror either toward the edge of the beam or farther away from the laser. The best illumination ratio is 4:1, but anything from 3:1 to 10:1 will work. The angular position of the mirror should be adjusted to illuminate the film uniformly.

"To make an exposure, turn on the laser and let it warm up for 15 minutes. Turn off the room lights, block the laser beam with a piece of black cardboard, open the film holder and after a few seconds unblock the laser beam. The laser must of course be enclosed by a housing that blocks all light except the beam. Do not touch the apparatus with anything that might cause the parts to vibrate, including the cardboard used for blocking the beam. Try several exposures of 30 seconds, one minute and two minutes. Develop the films in Kodak D-l9 developer for six minutes at 68 degrees Fahrenheit. Rinse, fix and dry according to Kodak instructions. Good holograms appear as transparencies that range from light gray to somewhat darker gray when they are examined in white light. The image can be reconstructed by looking through the hologram toward diverging rays from the laser. (It would be dangerous to look down the beam as it comes from the laser, but the diverging lens greatly reduces the intensity of the light.) Rotate the film around its vertical axis until the brightest image appears. The image can also be seen by looking toward a point source of ordinary light, but the quality will be degraded somewhat. A satisfactory source can be improvised with a 35-millimeter slide projector. Insert a piece of red gelatin in the space normally occupied by the slide and insert a pinhole mask over the front of the projection lens. The mask can be a piece of cardboard perforated by a hole about three millimeters in diameter. The perforation should be located at the center of the projection lens."

THE aneroid barometer, which displays changes in atmospheric pressure with a rotating hand actuated by a flexible closed chamber, can be found in millions of U.S. homes. Among household scientific instruments it is outnumbered only by the clock and the thermometer. Egon E. Muehlner, an engineer of Santa Ana, Calif., observes that barometers of this type are neither as reliable, as accurate nor as handsome as the contrabarometer, in which the changes m pressure are displayed by the level of fluid in a glass tube. Muehlner recently built a contrabarometer, one of the few working models in this hemisphere. He discusses its virtues and explains how to make one:

"The contrabarometer," writes Muehlner, "is basically a V-shaped glass tube that contains two fiuids. One arm of the U is closed at the top and holds a column of mercury that extends upward a short distance into the other arm, which is open to the atmosphere. The open arm, which is surmounted by a small bulb and a capillary tube, holds a column of brightly colored oil that rests on the mercury. Movement of the mercury m response to variations of barometric pressure causes a relatively large excursion of oil in the capillary [see illustration at right]. When the pressure rises, the meniscus of the oil falls, and vice versa. This action accounts for the name of the instrument. Contrabarometers adorned many living rooms here and abroad during the l9th century, providing the head of the house with a scientific foundation for his statement: 'It's going to rain.'

"The contrabarometer is significantly more reliable and accurate than the aneroid type, which tends to stick and to suffer from all the other ailments that beset mechanical contrivances. Moreover, when the geometry of the contrabarometer is properly proportioned, the instrument requires no temperature correction, as the conventional mercury barometer does.

"The construction calls for some glassblowing that, with a little practice, can be achieved by most beginners. Shops that specialize in the repair of neon signs also accept work of this kind. The construction begins with the acquisition of the glass tubing specified in the accompanying drawing [left]. The sizes are available from most distributors of scientific supplies. The substitution of other sizes will alter the scale factor of the instrument and the relative volumes of fluid required for temperature compensation. Appropriate dimensions that differ from those I specify can be calculated by means of the accompanying formulas [lower right ]. For the indicating fluid I use Meriam Red Oil D-2673, which has a specific gravity of .827 at four degrees Celsius and a vapor pressure of one torr at 25 degrees C. The oil is brilliant red. It can be obtained from the Charles Meriam Company, Inc., 5017 Telegraph Road, Los Angeles, Calif. 90022. Other oils can be substituted by taking their density and thermal expansion into account in making the calculations. Changes in the total volume of fluid should be accommodated by altering the size of the spherical bulb, which in my design has an inner diameter of 26 millimeters.

"My instrument is designed to be used at sea level. It has a scale factor of 10: the oil rises or falls one centimeter when barometric pressure changes one millimeter. The reference scale is graduated from 775 to 730 millimeters of mercury, a pressure range of 45 millimeters. At locations of higher elevation the instrument can be modified by reducing the height of the mercury column as measured between the upper and the lower meniscus of the metal.

"Before the instrument is filled the glassware should be firmly mounted on an attractive base. Much of the satisfaction of having a good barometer comes from its beauty. My base was made of 3/4-inch walnut. I gave it four coats of marine varnish and hand-rubbed it first with pumice and water and then with rottenstone and linseed oil. A friend who made a contrabarometer carved his base by hand from a block of oak. The carving features embellishments and ornaments in the style of the 1880's.

"The glassware is mounted on the base with three clamps, two at the top and one at the bend of the U. The clamps are preferably made of polished brass. A short length of slit rubber tubing is placed around the glass at the clamping points. The glass must not be clamped too tightly to the base because it might break if the wood warped. The oil capillary should clear the wood by at least 1.5 millimeters to leave space for inserting the scale.

"The barometer can be filled by either of two procedures. The easier one calls for a vacuum pump. If this technique is employed, the bulb at the top of the mercury column must initially include a short tube about seven millimeters in diameter. This tube is softened in a flame and constricted in a narrow zone to a diameter of about four millimeters just above the point where it will be sealed into the bulb. Air is pumped from the bulb through the tube, creating a vacuum that draws mercury into the system. The instrument requires approximately 150 grams of triple-distilled uncontaminated mercury. Impure metal will stick to the glass. Mercury of the required quality can be obtained from a distributor of chemicals or perhaps a dentist. With the instrument in the upright position connect the vacuum pump to the exhaust tube and start the pump. Place a dishpan of water under the barometer to catch metal that may be spilled accidentally. Pour mercury into the open arm of the U with a small funnel. Continue adding mercury carefully until the meniscus rises to a height of 29 millimeters in the bulb at the top of the mercury column. The metal will then stand at a height of 26 millimeters in the 15 millimeter tubing of the other column. The vacuum pump should be capable of reducing the pressure to less than .05 torr. Let the pump operate for 15 minutes after the barometer has been filled. Then seal off the tube through which the air was exhausted by heating the constriction to the temperature where the glass softens.

"The barometer can also be filled without a vacuum pump. Procure from a distributor of scientific supplies a 10-foot length of size 'A' Intramedic polyethylene tubing, a product of the Clay-Adams Company. This material is manufactured for surgical procedures. It has an outside diameter of .038 inch and a bore of .023 inch, and it fits a No. 23 gauge hypodermic needle. Lubricate the exterior surface of the plastic tubing with the Teflon suspension Slip Spray, a product of the Du Pont Company. Thread the tubing through the barometer until the end of the plastic capillary rests inside the bulb at the top of the mercury column. The bulb should be sealed. If difficulty is encountered in inserting the plastic capillary, use a leader of fine music wire before sealing the bulb. Rest the instrument on its edge with the sealed bulb lowermost. Elevate the opposite end about 30 centimeters. With a small improvised funnel pour mercury through the plastic capillary and into the bulb. Avoid trapping an air bubble in the bulb at the point where it joins the glass tubing by elevating the opposite end still more as necessary. Lower the elevated end slowly as the tubing fills. Add mercury until the metal half-fills the U bend. Remove the plastic capillary. Rotate the instrument to the upright position. Mercury will rise in the open tube, creating a vacuum in the bulb at the closed end. With an eyedropper add mercury to the specified level. Avoid spilling mercury. The fumes can be toxic.

"Oil is added on top of the mercury. Through the glass capillary insert an appropriate length of plastic capillary tubing and push the end into the glass sphere. Oil can be siphoned into the system, but it flows slowly. The job can be speeded up by forcing oil through the plastic capillary with a hypodermic syringe. Attempts to pour oil directly into the glass capillary fail. Bubbles of air become trapped in the narrow bore. Fill the capillary to the point where the indication of a specific barometric pressure is desired. If this is to be accomplished, the current barometric pressure must be known. I learned the current pressure by telephoning the local airport. Airports conventionally report barometric pressure as the 'altitude setting' in inches of mercury. I placed a bit of adhesive tape on the glass capillary at the point where the upper edge of the tape coincided with the meniscus of the oil. The tape thus marked the current barometric pressure. Inevitably a few air bubbles will become trapped in the oil capillary. To get them out rotate the barometer slowly in the counterclockwise direction. This motion gradually lowers the bulb at the closed end of the instrument. Metal drains into the bulb and simultaneously lowers the level of oil in the capillary. Eventually all the oil will drain into the sphere, where the bubbles will break. When they have broken, restore the instrument to the upright position.

"One must now determine the scale factor of the instrument: the distance in millimeters that the oil meniscus is displaced by a change in barometric pressure of one millimeter. By the application of the formula I designed my instrument for a scale factor of 10. Glass tubing, however, is not manufactured to close tolerances. My instrument turned out to have a scale factor of 10.7, as measured by a water manometer. To calibrate the instrument I connected one end of a U-shaped glass tube partly filled with water to the open end of the barometer. The other end of the water manometer was fitted with a short length of flexible hose and a pinch clamp. I blew into this hose (or sucked it) and clamped it. I then measured the resulting displacement of the water. The pressure thus applied to the barometer, measured in millimeters of mercury, is equal to the displacement of the water, also measured in millimeters, divided by 13.55. A series of calibration points was established and marked on the glass capillary with bits of adhesive tape by the similar application of other pressures to the water manometer.

"I made the permanent scale of my instrument from a strip of white Formica, the plastic sheeting used for covering kitchen counters. The principal graduations were applied with black adhesive tape 1/16 inch wide and the subdivisions with tape 1/32 inch wide. Numerals were applied with transfer sheets. These materials are available from dealers in art and drafting supplies. The scale was calibrated both in millimeters of mercury and in millibars, after which it was positioned according to the tape tabs and fastened to the base with brads. One inch is equal to 25.4 millimeters of mercury, a millimeter of mercury (one torr) is equal to 1.333 millibars and a millibar is equal to a pressure of 100 newtons per square meter. I equipped the instrument with a sliding fiducial point for observing relative changes in pressure with respect to time and also mounted a thermometer in the center of the base.

"One word of caution: If you make the contrabarometer, be careful when you move it from place to place. If it is not held upright, fluid can easily spill out of it and even spoil the vacuum at the top of the mercury column. If anyone has any question about the making of the instrument, my address is 13882 Dall Lane, Santa Ana, Calif. 92705."

THERMAL engines of unusual design have been described in these columns from time to time. J. M. Clarke, an engineer of Basingstoke, England, now challenges anyone to devise a simpler heat engine than one he recently built. Clarke's engine has only one moving part: a metal tube with weights at the ends that can be adjusted radially for balancing the mass with respect to its long axis. The tube rolls on a pair of level rails and is powered by a pair of candle flames [see illustration at left].

"The weights," writes Clarke, "exert a bending moment at the center of the tube. Four nuts, spaced at equal radial distances around the weights, can be adjusted to compensate for slightly bent or out-of-round tubes. The tube is preferably of thin-wall aluminum at least half an inch in diameter and 24 inches long. Heat causes the tube to expand in the center and bow toward the opposite side. Gravity causes the bowed tube to roll away from the source of heat. The motion can be maintained by pushing the candles forward as the tube advances. Each element of the tube near the candles is subjected to an alternating stress modified by the thermal expansion. The tube absorbs heat on the side that expands and loses it on the opposite side. Clearly the device converts some portion of the heat into mechanical work that accelerates the rod. In practice I have found that the performance of the engine is limited by its poor capacity for losing heat. I have succeeded in measuring the power output by attaching one end of a length of twine to the tube and a small weight to the other end. The twine coils around the rotating tube and lifts the weight."

Roger Hayward, who illustrates this department, suggests a way of making Clarke's engine fully automatic. He would place a small tank of water between the rails, perhaps a tank made of aluminum foil. The candles would ride across the water in an aluminum boat. The boat could be propelled by a short mast at the bow that engaged the advancing edge of the tube. Those who accept Clarke's challenge can reach him at Forge Cottage, Dogmersfield, Basingstoke, Hampshire, England.

Bibliography

COFFEE-TABLE HOLOGRAPHY John Landry in Journal of the Optical Society of America, Vol. 56, No. 8, page 1133; August, 1966.

HOLOGRAPHY FOR THE SOPHOMORE LABORATORY. Robert H. Webb in American Journal of Physics, Vol. 36, No. 1, pages 62-63; January, 1968.

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