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Transit Telescope

A Transit Telescope is the basic instrument for determining time by stars. It is fixed such that the instrument rotates on its axis in a north-south plane.1 An observer using the transit telescope watches a chosen star pass across the cross-hairs. These time measurements probably provided time base for the Haish telescope in the Goodnow observatory and did provide time for the Rock Island railroad. This Fauth Transit Telescope was purchased by the college in 1888 for a concessional price of $550.   The transit telescope (in Noyce Hall foyer), chronograph (in display case F2), astronomical Seth Thomas clocks (in Kistle Science Library), and micrometer (in case F2) were purchases requested by Samuel J. Buck following the construction of the Goodnow observatory with its Haish telescope. “If a man makes you a present of a fine horse, immediately you begin looking about for a saddle and bridle, harness, carriage and sleigh in order to make use of the horse,” The transit telescope is the basic instrument for determining time by stars. The axis of a transit telescope is fixed in the east west direction so that when the instrument moves, it moves only up and down in a north-south plane. This one was mounted on a Y shaped metal base and located in the brick transit house just East of Goodnow. It was reported to have an objective with aperture of 3 inches and three eye pieces, a direct, diagonal, and a nadir, with approximate powers of 50, 100, and 30, respectively.  An observer would watch a chosen star pass across the cross-hairs. The time between one passage of a star and the next is one sidereal day. The interval between two successive passages of the sun past some fixed point in the sky, such as the meridian, is one solar day, and it is 3 minutes 56 seconds longer than the sidereal day. Because the observations of stars are made at night, the cross-hairs must be illuminated. The cross-hairs are in the center of the telescope tube, and a window at each end of the support admitted light from a kerosene lamp to make the cross-hairs visible. From a knowledge of the longitude of the observatory and the location in the sky of a particular star, one can calculate precisely the time at which that star passes the local meridian. If the passage is observed with the transit telescope, the derived time can be used to set a clock or, more likely, produce a correction to be applied to the clock’s reading. It is easier to use corrections than to set clocks frequently.

Jolly Balance

A Jolly balance has a weak spring so that it stretches a great distance when a small force is applied. If a small, known force was applied to the pan and the resulting extension of the spring noted, the spring constant could be calculated and the balance then used to measure other small forces. The name comes not from the attitude of its users but from the name of its 19th century inventor

Magdeburg Hemispheres

In 1654, Otto von Guericke, burgomaster of Magdeburg, performed an experiment using two brass hemispheres which fit together closely to make a sphere and which could be evacuated with a vacuum pump, which von Guericke had invented in 1650. Von Guericke used hemispheres about fourteen inches in diameter, and when as much air as possible had been pumped from them, two teams of horses could not pull the hemispheres apart. The hemispheres shown here were purchased for $6.00 in 1885. __________________________________________ Mayor Otto von Guericke of Magdeburg (1602-1686 AD) clearly had a flair for the dramatic. His scientific demonstrations involved props such as guillotines and strongmen. But his most famous public experiment at Regensburg sometime around 1654 (the exact date is uncertain) included what came to be known as the Magdeburg hemispheres. Made of copper or brass, the hemispheres can be joined to form a hollow globe. Using an air pump (which von Guericke also invented), he removed the air from the sphere and showed how 16 horses – 2 teams of 8 each – could not pull the halves apart. The sphere immediately fell apart once air was reintroduced. From this experiment, he showed that the air pressure surrounding the hemispheres, without the counteraction of the pressure normally existing inside the sphere when it was filled with air, made them cling together. Scientists were just beginning to realize that we live under an ocean of air, with the mass of the atmosphere corresponding to a pressure of about 1 kg per cm2. The discovery of the sheer force of the pressure of the atmosphere led to the development of the first steam engines in the 1700s. Although the 1600s were a tumultuous time in Magdeburg’s history, von Guericke still found time to contemplate various questions about the nature of space. Aristotle (384-322 BC) proposed that “nature abhors a vacuum,” currently defined as any volume with a lower particle density and gas pressure than the surrounding atmosphere. This postulate would be believed for almost 2000 years. Evangelista Torricelli (1608-1647 AD), one of von Guericke’s contemporaries, demonstrated in 1643 that a vacuum could exist in space above an enclosed column of mercury. However, from astronomical observation of the constancy of the time it took for planets to revolve, von Guericke concluded that space is also a vacuum without friction. He also conducted experiments on the elasticity of air, as well as the relation of air pressure and altitude. Combined with Blaise Pascal’s discovery of the link between atmospheric pressure and weather, von Guericke proposed meteorological stations to gather data to forecast the weather. Other discoveries he is credited with include the magnetization of iron, the invention of a static electricity generator, and the observation of colored shadows. -Mira Lamb 2018 References “Guericke, Otto Von,” accessed April 14, 2017. https://www.accessscience.com:443/content/ guericke-otto-von/m0091124. Marquardt, Niels. “Introduction to the principles of vacuum phsyics,” last modified November 22, 2016. https://cds.cern.ch/record/582156.

Nicholson's Hydrometer

The Nicholson hydrometer is used to determine the specific gravity of a material more dense than water. The conical cup at the bottom should have enough lead in it that the cylinder floats upright in the water. An index mark is made on the the floating tube above the water level. Then three measurements are made: (1) weights are added to the top pan to press the float down to the index mark; (2) the substance whose density is desired is placed in the top pan and weights are added to depress the float to the index mark again; (3) the substance being examined is placed in the bottom pan and again weights are added to the top pan to bring the float to the index mark. From the values of the weights added in the three conditions the density of the substance can be computed.

Vacuum Pump and Bell Jar

This vacuum pump (hand operated by a handle now missing) was made by Jas. W. Queen & Co. and purchased in 1885 for $25. _________________________________________ In 1654, Otto von Guericke was credited for inventing the first Vacuum pump. It was not until 1660 that Robert Boyle published his New Experiments in which he describes his theory of air pressure (Brush, 13). His theory would then become known as Boyle’s Law in which the volume and absolute pressure of a contained gas are inversely proportional. Until the 17th century, air was an invisible substance that could not be studied or explained. With the invention of the Vacuum pump, the principles of air could not only be studied, but they could be demonstrated in such a way that was ultimately undeniable (Brundtland, 265). This type of demonstration not only depicted scientific discovery, but also commanded a certain amount of excitement. Well into the late 1700’s and early 1800’s, experiments continued in the realms of sound, electricity, and mechanics (Brundtland, 267). Grinnell College bought a vacuum pump in 1885 for 25$ from a fairly well known manufacturer of scientific instruments known as Queen & co. This pump is a single barrel pump with a standard sealable bell jar. Air can be evacuated from the jar by pumping the handle (now missing) up and down. -Jordan Hamilton 2019 References Brundtland, T. "Francis Hauksbee and His Air Pump." Notes and Records of the Royal Society 66.3 (2012): 253-72. JSTOR. Web. Brush, Stephen G. "Gadflies And Geniuses In The History Of Gas Theory." History of Modern Physical Sciences The Kinetic Theory of Gases (2003): 421-50. JSTOR. Web.

Nicol Prisms

A Nicol Prism is a device to produce polarized light. It is made from a crystal of calcite, which is cut along a precisely determined plane and then cemented back together with Canada balsam. When a beam of light enters the crystal along the line defined by the mounting of the crystal, it is broken into two beams. In the two beams the planes of vibration of the electric field vector are perpendicular to one another. When the two beams strike the cemented cut, one is passed through and the other is deflected to the side and absorbed in the mounting.

Laser

This is the first laser owned and used by the Grinnell College Physics Department. It is a helium-neon laser, and the gas inside the tube was excited by a radio-frequency field produced by an oscillator. This laser was purchased by Bob Noyce, who experimented with it for a short time and then brought it, carried on his lap on the airplane, to Grinnell and presented it to the Physics Department. The orange plastic cover is lying behind the instrument.

Lens on Stand

This lens on a ball mount has been used for many years for lecture demonstrations. Although neither date of purchase nor maker is known, it certainly has been part of the college's equipment since the early 1900s if not earlier.