Grinnell College Physics Museum

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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

Rotator with Governor

Table and all attachments cost $40. Apparatus includes rotator table, Rotator part with brass circles to show shape of rotating earth, other Rotator part demonstrating centrifugal force (like Watt governor), Doppler whistle, and Rotator crank wheel.

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.

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.

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.

Atwood's Machine

Atwood's machine is a device to observe the acceleration of a moving system acted upon by small forces. Two unequal masses are hung over a pulley by a light cord. The pulley turns with low friction, and the acceleration of the system of masses can be measured. The mass accelerated is the sum of the two masses on the string, and the force producing acceleration is the difference in the weights of those two masses. In this device, the pulley carrying the cord turns on roller bearings, like those designed by Atwood in the 17th century. The second image shows the low-friction roller bearings