Comments about Magdeburg Hemispheres Reenactment ‡
M.H. Hablanian and C.H. Hemeon
A novel event was part of the 30th Anniversary celebration of the American Vacuum Society in 1983: On October 22, 1983, Boston’s Museum of Science and the New England chapter of the AVS recreated von Guericke’s famous Magdeburg Hemispheres experiment. This activity was organized in association with an exhibit of historical vacuum apparatus that was first displayed at Boston’s Museum of Science before being moved and displayed at the 30th AVS Symposium in Boston. A video tape of the reenactment was shown repeatedly on a television monitor at the exhibit.
Six horses, three on each side, pulled on evacuated steel hemispheres. Briefly, the first three attempts to separate the hemispheres were not successful, but on the fourth try, the horses were able to separate the two halves of the chamber.
Most comments overheard at the exhibit from people watching the proceedings on the screen were that the separation was due to a lateral shift of one side relative to the other. Specifically, this explanation is incorrect because lateral restraints were incorporated internally into one of the chambers to inhibit shifting due to a possible uneven transient load during an initial jerk. The explanation for separation lies in the fact that an impulse load developed by the horses was actually great enough to overcome the force of the atmospheric pressure.
Even though the demonstration in Boston was done in the spirit of fun, a number of technical problems had to be considered during the design of the modern version of the hemispheres. They may be of some interest to the readers.
The secret and the drama of the demonstration lies in the question of coordination of the effort of the two teams of horses as well as the simultaneous application of the pulling force by each horse. If von Guericke’s experiment were to be scaled down to only one horse (with a smaller chamber) and if one side of the evacuated chamber were attached to a tree, the horse would have separated the chambers each time. von Guericke may have been a better showman than a scientist, but, to be fair, it must be noted that in this time Newton’s laws were unknown; force and momentum were usually confused and energy considerations in impulse load calculations were not appreciated.
As designers of the modern hemispheres, we were so acutely aware of the possibility of embarrassment despite the use of only three horses on each side that safety chains were provided to prevent injury to the horses or to their handlers in case the half-chambers flew through the air after separation.
The following considerations may be illustrative. von Guericke usually used eight horses on each side and hemispheres of 20 in. in diameter. The shape of the chambers is, of course, immaterial. Only the projected force acting on the area enclosed by the sealing circle should be considered. This gives an atmospheric pressure force of 4600 lb and the challenge of only 575 lb per horse. It would seem that even Don Quixote’s Rosinante should have been able to produce that much force. As a rule of thumb, a horse can produce a pulling force (standing on soft, grassy ground) roughly equivalent to its own weight. 1 von Guericke undoubtedly was clever enough to use light-weight carriage horses. At the Boston demonstration, two teams of three horses each were used, each horse weighing over 1600 lb.
1 The Draft Horse primer, Maurice Telleen (Rodale Press, 1977)
There was another important distinction between von Guericke’s and our demonstrations. The more horses are used, the more difficult it is to have them pull in unison and, more importantly, to have the two opposing teams apply a peak force simultaneously. The horses used in Boston (Eastern Draft Horse Association) are trained for and accustomed to participation in pulling contests. They usually produce a great exertion for about 10 s pulling continuously a loaded sled approximately 30 ft. They seem to be trained to move initially with sudden acceleration, presumably to overcome the static friction force and dislodge the sled. When given a signal, they practically leap forward, producing a substantial impact force when the chains connecting their harnesses and the hemispheres become taut. In the photograph taken at the site, one of the horses appears to have only one hoof on the ground. If we make what would appear to be conservative assumptions for acceleration, velocity, and time of interaction, we can estimate that the impact force can easily exceed the force of atmospheric pressure.
The chambers used in Boston were 24 in. in diameter. We did not want to make them larger for two reasons. First, to be reasonably close to the original (20 in.), especially because of smaller number of horses used; second, because we wanted the hemispheres to be light enough for handling during the demonstrations. We used two 0.25- in.-thick, deep-dished steel pieces without flanges. A commercially available bell-jar gasket was used as the seal.
The nominal force of atmospheric pressure for a 24-in.-diameter sealing circle is about 6550 lb. Separation is likely to occur with a force somewhat lower than that because the gasket between hemispheres may start leaking when most of the atmospheric force is counter-balanced by the pulling force. We used the 0.25-in.-thick chamber edges for sealing, without flanges, giving approximately 350 psi initial sealing pressure on the rubber gasket. von Guericke must have had difficulties with the seal because he used a greased leather gasket and rather thick flange with a wide sealing surface, giving an initial sealing pressure roughly one quarter of ours. In the model displayed in the museum, the flange surface was ~ 1 in. wide.
One final comment: Why was the gasket propelled into the air after separation of the chambers? (see figure). One possibility is that the adhesion between the gasket and the edge of the mating chamber was stronger than the adhesion to the edge to which the gasket was attached. But, more likely, the air pressure distributions around the L-shaped gasket at the time of separation may have produced a net outward force.
In conclusion, the lessons from the recreation of the Magdeburg experiment are as follows:
1. Use small, tired, uncoordinated horses. 2. Feed the horses as little as possible two days before the test. 3. Build in some cushion into the harness. For example, use nylon straps or ordinary ropes instead of chains. 4. Tell the drivers not to synchronize the forward signals to the two teams. 5. If money is no object, use a cylinder with a well-sealed piston instead of hemispheres to eliminate the effect of impact forces. 6. Finally, always use a safety chain or strap to keep things together in case of separation.
Acknowledgments: John Sullivan (MKS) conceived the idea of the demonstration and participated in all phase of the project. Ted Madey (NBS) set the project into motion and obtained the necessary funds from the AVS and its New England chapter (with the help of Chairman Tom Shaughnessy). Larry Bell of Boston’s Museum of Science organized the demonstration, obtained the teams of horses, provided the television tape, and served as Master of Ceremonies. Cal Hemeon designed the details and supervised the construction of the hemispheres.
‡ This account has been edited by T.E. Madey. The original was published in “History of Vacuum Science and Technology”, eds. T. E. Madey and W.L. Brown (American Institute of Physics, 1984) p. 61-63.