Voss Machine
Static Electricity
<p><strong>In this machine a fixed plate of glass has two metallic foil sectors glued onto it. The slightly-larger rotating plate has six foil dots with raised studs glued to it; the foil sectors are large enough so that two of the studs are opposite each sector. </strong><strong>In the middle is a horizontal ebonite rod, with collecting combs on the ends that are connected to Leiden jars. The vertical brass rod has metallic combs attached to its ends. The crank sets the machine in motion. At the northeast and southwest corners there are fixed metallic brushes that are in electrical contact with the foil sectors of the fixed disk. This item is an excellent match for a machine in the ca. 1900 Max Kohl catalogue.Its cost was about 70 Marks, depending on the diameter of the disk. </strong></p>
<p><strong>This machine may also be refered to as a Toepler-Holtz machine.<br /></strong></p>
Property of the W&L Physics & Engineering Department
Absorption Cell
Optics
<strong>These glass spheres were used to contain gases such as iodine, bromine, and hypo-nitrous acid in order to study their light-absorption properties, i.e. their absorption spectra. Light shown on the cell passed through the glass and was absorbed by the gas differentially at different wavelengths. The spherical cells are listed in the 1888 "Illustrated Catalogue of Instruments used in Physical Optics" published by James W. Queen and Co. of Philadelphia at $7.00.</strong>
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Rotator
Mechanics
<strong>Many pieces of demonstration equipment must be rotated to show a desired effect. A Rotator is a device with a crank that can rapidly rotate a small horizontal platform about an axis. Other pieces of equipment, like Newton’s Color Wheel, attached to the platform of the Rotator for the purpose of demonstration.</strong>
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Ammeter
Electrical Measurements
<strong>An ammeter is a device for measuring electric current. The analog ammeter is constructed so as to pass only a small (but known) fraction of the current to be measured through the meter movement that deflects the needle. This allows a very sensitive measuring device to safely measure large currents. The basic mechanism was developed by Edward Weston in the last few years of the 19th century. Analog ammeters have been almost entirely replaced in the last two decades by digital current meters.</strong>
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Arc Light Control
Electricity
<strong>Lightning is the ultimate arc lamp, and so Benjamin Franklin's 1752 experiment of drawing electricity from the clouds and jumping a spark is perhaps the first arc lamp. In 1801 Humphry Davy observed the brilliant spark obtained when the connection between two carbon rods, attached to the poles of a battery, was broken. Some years later, in a demonstration lecture at the Royal Institution, he produced an arc nearly three inches in length. He used a voltaic battery with 2000 sets of plates, each four inches square. Commercial arc lighting had to wait for the development of dynamos such as the Gramme Machine in the early 1870s.</strong>
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Bladder Glass
Pneumatics
<strong>This is a device used to demonstrate the effect of atmospheric pressure. A glass vessel with openings at the bottom and top has its large top opening covered by a piece of animal bladder that is tied to seal it around its edges. As air is pumped from the vessel through the bottom opening, atmospheric pressure causes the bladder to deform downward, ultimately bursting with a loud bang. This seemingly simple effect is mentioned in the majority of nineteenth century physics textbooks.</strong>
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Thacher's Calculating Device
Calculating Devices
<strong>This early relative of the slide rule has a cylinder 4 in. in diameter and 18 in. long that rotates and slides in and out between the array of twenty equally-spaced parallel bars. The bars have a scale with numbers spaced logarithmically, as on a slide rule’s multiplication scales. The cylinder also has a logarithmic scale engraved lengthwise on it. The net effect is that of a very long slide rule, allowing calculations accurate to four to five digits to be made. In 1934 this instrument was sold by Keuffel and Esser for $80.00.</strong>
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Current Balance
Electrical Measurements
<strong>A current balance measures the force of repulsion between two wires, each carrying an electric current. An upper wire is fixed in place, and the wire directly below it is free to move. The currents are adjusted so that the electrical attractive force on the lower wire perfectly balances the gravitational force on the wire so that the wire levitates, at rest. Weights can be added to the lower current to increase the gravitation force. This device allows one to study the attractive electrical force between the wires as a function of the currents through the wires. Lord Kelvin’s original current balance design, used in the pictured device, dates from 1882. The current being measured passes through parallel circular loops of wire that each carry the same amount of current.</strong>
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Differential Thermopile
Heat
<p><strong>NOT YET LOCATED. </strong></p>
<p><strong>When two wires made of different metals are connected to make a loop, and the two junctions between the wires are held at different temperatures, a voltage is produced and electrical current can be detected with a sensitive current meter. This is the Seebeck Effect, discovered by Thomas Seebeck in 1821. The effect is multiplied when there are more than two junctions in series in the circuit, with alternate junctions in close thermal contact with each other. By measuring the current, one can measure the temperature difference of the junctions. The Differential Thermopile was invented by Macedonio Melloni (1798-1854), an Italian physicist who worked in France and Italy. Melloni's research dealt with thermal radiation, and he developed the thermopile to make quantitative measurements of the intensity of the radiation. Washington and Lee’s differential thermopile was made by E.S. Ritchie. The model with 20 pairs of junctions sold for $25.00 in the 1881 catalogue; for $40.00 you could get the 49 pair model.</strong></p>
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Electric Whirl
Static Electricity
<strong>In the nineteenth century almost all power supplies were direct current (DC). In order to step voltage up or down, the direct current had to be converted to alternating current (AC), stepped using transformers, and then converted back to DC. The standard technique developed for changing DC to AC was a mechanical interrupter or Electrotome, run at a frequency controlled by the mechanical attributes of the device. This interrupter was made by Morris E. Leeds & Co of Philadelphia, "Manufacturers of High Grade Electrical Measuring Instruments, X-Ray Equipment and other Scientific Apparatus". The date is no later than 1900-1902.</strong>
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