The Tamarack Mine Experiments
The mine shafts which existed in the vicinity of the College provided a unique opportunity for physics research. This was particularly so for the Tamarack Mine shafts which were vertical and almost a mile deep. In 1901 the Physics faculty (McNair, Fisher, Osborne, and Grant) along with John B. Watson, chief engineer, and George Slock, assistant engineer, of the Tamarack mining company,1 began experiments using long pendulums (pendula) as plumb bobs in the #2, #4, and #5 shafts of the Tamarack Mine. The goal of these measurements was to transfer a reference line from the surface to aid future horizontal drilling operations. Being physicists, one of the first results which shows up in the lab notebook is the period of the pendulum, a result which is largely irrelevant for use as a plumb bob.
The first pendulums were made with #24 steel piano wire and 50 pound cast iron weights and were hung 4,250 feet down shaft #5. The period of these pendulums was 70 seconds. The weight actually stretched the wire about 15 feet. For some measurements the weights were placed in oil or water to help damp the motion though this was insufficient to completely stop the motion.2 They typically used multiple measurements of the oscillating pendulums “as in the method of determining the zero point of a balance by observing the oscillations of its pointer,” rather than waiting for the motion to stop.
The first and most well-known result was from their first measurements—that the distance between two adjacent pendulums at the bottom was slightly larger than at the top (about 1 inch out of 15 feet). This was at a time when the fruits of the industrial revolution were just making it into people’s homes and many people had a great interest in science and the promise it had for their future. Thus it is not too surprising that a detailed account these early results appeared in the local newspaper, the Daily Mining Gazette (Oct. 8, 1901). The reporter suggested that possibly some new phenomenon had been discovered. The results received national attention when the story was picked up by several other newspapers resulting in considerable speculation about the cause for this unexpected result. As one might expect, included were some rather unorthodox explanations. In fact, one can find these first results still cited even today (usually incompletely, incorrectly, and/or with significant creative embellishments) as evidence for a hollow Earth,3 government conspiracies, coverups, UFOs, and/or for the general failure of Newton’s law of gravity.
In the introduction of his 1902 article in The Engineering and Mining Journal,4 where the full plumb bob experiments are reported, McNair provides a rather low-key response to the hoopla:
The publication … in the Portage Lake Mining Gazette of the fact that a divergence in the plumb lines had been observed, attracted considerable attention, and brought forth many attempts to explain its existence. Those most familiar with the conditions had no satisfactory theory to offer. To them it was evident that more data must be secured before the cause could be correctly assigned.
McNair considered only two of the many explanations to be worth mention—a gravitational attraction to the walls of the shaft and magnetic effects. The gravitational effect was easily shown to be much too small. The group proceeded to use different materials for the weights and wire, as well as different positions, ventilation conditions, a pair of 4,440 foot pendulums in shaft #4, etc., for additional measurements.5 In some cases the pendulums were closer together at the bottom, though in most cases they were farther apart and there was no difference seen between the use of magnetic and non-magnetic materials. Based on a very strong correlation with ventilation conditions, they concluded that the effect was entirely due to the significant (natural) airflow in the mine shafts.
The group received national attention again after they tried dropping two, 2″ diameter steel balls down one of the shafts. They had estimated that the balls should strike a point approximately 4 feet eastward of straight down due to the Earth’s rotation. For both attempts, however, the ball never even made it to the bottom. While the work was done one afternoon by the curious just to see what would happen, and was not published, the result was picked up and then exaggerated by the popular press who stated that anything which was dropped down such a shaft would end up on the eastern wall whether it be a wrench or human body parts (Full Quote [PDF]). While he initially ignored these reports, in 1906 he finally had had enough and McNair wrote:6
Instinctively one feels that … very far underground conditions must be quite different from those one finds on the surface… When, therefore, there is made the statement that objects dropped into the deep shafts of the Lake Superior copper district ‘do not fall to the bottom but are invariably found clinging to the east side of the shaft’ probably the average reader is apt to credit the statement as noting one more of the strange and uncanny facts which belong to the underworld…. When the press correspondent in telling it adds a touch of the gruesome in the implication that among the objects frequently found ‘clinging to the east side of the shaft’ are the pieces of a dismembered human body one may suppose that reportorial skill has reached its acme… Such a story was put forth not long ago … and was copied in the daily press throughout the … country. One may question whether a plain statement of the truth could obtain more than a fraction of the same extent of circulation. (Full Quote [PDF])
McNair concluded that air resistance and air currents had a large effect on these experiments, a result which is not too surprising.7 Since there was no way to remove the air, these experiments were not pursued.
The group then set about performing very careful and extensive experiments of the acceleration due to gravity, g, in the mines using calibrated “half-second pendulums” at various locations around the mine.8 The work was a collaboration with John F. Hayford, Inspector of Geodetic Work at the (U.S.) Office of the Coast and Geodetic Survey. McNair’s principle motivation was to use the small variations in g to aid in determining an improved value for the density of the Earth and, of course, to ultimately use the technique to find ore deposits. Extensive measurements were made, sent to the Geodetic Survey for analysis, and after considerable delay the corrected values of g for the first measurements were returned. A fractional increase in g with depth of 7.7 × 10-5 per km was found.9 Based on a simple analysis, a decrease was expected. McNair was still questioning the calibration of the pendulums, shipped from Washington D.C., almost 10 years later, and was apparently concerned about the increase in g with depth they observed. He received reassurance from Hayford in 1911 (then in Evanston) that the calibration simply could not have been that far off. Nevertheless, after the press the group received for their previous measurements, it is understandable that they were reluctant to publish any additional controversial results. This was unfortunate since their results could have provided some of the earliest evidence for Earth’s very dense core had they been published.10
Any relationship between the values of g obtained and the various local geological formations was never determined. According to Fisher, this would “involve a great amount of computation, promised by the U.S. Coast and Geodetic Survey” which apparently was never carried out.
Shortly after these experiments Osborne left to begin a long and successful career at the Bureau of Standards in recognition of which he would later return to receive an honorary Doctorate.
1. Both Watson and Slock had Engineer of Mines (EM) degrees from the Michigan Mining School: Watson in 1895 and Slock in 1896.
2. The extra buoyancy due to the oil caused the wires to shorten about 25 inches, a result which was considered “rather striking.”
3. When asked of the “hollow Earth” hypothesis in 1943, Fisher said the idea was “too absurd for serious scientific thought.”
4. Vol. 73, pages 578-580, (1902). A shorter version can be found in Science, Vol. XV, p. 994-6, (June 1902). Note that by this time, the Portage Lake Mining Gazette had changed its name to the Daily Mining Gazette, however McNair, as well as others, sometimes referred to it by its old name in this and several future references.
5. During winter of 2004/2005, these pendulums were recognized by Guinness World Records as having set the record for the World’s Longest Pendulum.
6. The quote here is from McNair’s original typewritten manuscript dated 1 June 1906. The article appeared, with minor editorial changes, in the Mining and Scientific Press, July 1906.
7. A calculation accessible to upper level physics majors shows that the balls would already be very near terminal velocity after the first 1000 feet. Terminal velocity occurs when the forces due to gravity and the forces due to air resistance balance.
8. They used Mendenhall 1/2-second pendulums capable of detecting relative changes in g of about 1 ppm (~ 1 milligal). By definition a half-second pendulum takes one second for a full swing. A summary of the experiments is in the 21 Sept 1902 issue of the Sunday Mining Gazette.
9. From a draft report to the Superintendent of the Coast and Geodetic survey, Wash D.C., by John F. Hayford, 8 Feb 1904 with corrections from 16 July 1906, MTU Archives MTU-023, box 2, Fred W. McNair Papers.
10. Their result is comparable to modern world-wide averages inferred from other types of Earth density measurements. For example refer to the “Preliminary Reference Earth Model” put forward by Dziewonski and Anderson in 1981.