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CENCO E/M Apparatus |
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This classic e/m apparatus captures the best features of the old device, while incorporating new technology to improve performance, safety and longevity. The apparatus consists of a specially designed vacuum tube supported at the center of a pair of large Helmholtz coils. Within this tube an electron gun, surrounded by a coaxial anode containing a single axial slit and composed of a filament parallel to the axis of the coils, produces a narrow beam of electrons, the paths of which are rendered visible by mercury vapor in the tube.
Similar to the classic version, the new tube has been equipped with a safer, more reliable keyed plug and its support system has been redesigned to make axial rotation easier, with the addition of a new end stop system to define the permitted rotation angle.
The beam normally impinges upon the side of the tube, but when current flows through the coils, the resulting magnetic field causes the beam to move into a circular path, the radius of which decreases as the current, and therefore the magnetic field, increases. By adjusting the current until the sharp edge of the beam coincides with the edge of any one of five bars spaced at different predetermined distances from the anode slit, the radius of the path becomes accurately known, leading to the computation of the value of e/m.
The filament of the e/m apparatus is under gentle spring tension to keep it in alignment with the anode slit as emission temperature is reached. Reference posts for indicating beam diameter fluoresce brightly during operation to facilitate exact setting of the beam in a darkened room. With an average life in excess of 100 hours, the e/m apparatus will last through years and years of classroom use. The experiment itself requires less than 15 minutes of operating time. A fine adjustment knob is also featured to control the filament current that is connected inline with a fuse, so that the bulb cannot be damaged when too much current is applied. The maximum current the filament can handle is 4.5 amps.
The old open wiring and binding posts have been upgraded to a completely enclosed system with labeled plug sockets that improve safety and reduce wiring errors. With the new model, you do not have to worry about blowing out the tube. The filament circuit is now protected by an easily accessible fuse to eliminate expensive tube damage due to excessive filament currents. The filament circuit can take AC or DC voltage due to a built in rectifier. The current can be controlled on the unit with a rheostat that is in line with the fuse mentioned earlier. The analog display shows currents from 0–5 Amps in increments of .2 A.
The Helmholtz coils are larger than any others on the market, sized at 66cm diameter. Wound inside rigid aluminum guide rings, the coils are supported with the tube on a hinged secondary base. In this new apparatus, both the support and connection systems have been upgraded. A gas strut improves the ability to change the height of the apparatus and allows for easy angle modifications so the unit can be orientated with the dip angle of the Earth’s magnetic field. The length of wire in each individual coil is 72 turn per coil using 149.04 m of wire.
The e/m tube is 18cm in diameter, much larger than other e/m tubes. Its frame is constructed of durable, dimensionally stable, high density HDPE polyethylene frame that won’t warp. This new frame replaces the wooden support frame of the older model, and it also incorporates both a sturdier hinge and a smoother double clamping system.
The following power supplies are required and available separately: power supply for the filament - WLS1799-17, for accelerating voltage - WLS1799-17, and for the helmholtz coils - WLS1799-13.
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- Monday, June 20, 2011 Mr. Reviewed By: Morris (Des Moines, Iowa) I purchased this apparatus to replace the very old similar e/m equipment that used vacuum tube power supplies. The new apparatus worked very well. I was able to get the dip angle from the internet, hook up the new power supplies, a few multimeters and fire it up in about an hour. The beam was easy to see in a dark environment and the experimental value of e/m that I calculated the first time I used it averaged out to be less than 3% different from the accepted value. It is an exciting piece of equipment to use. It clearly shows how a beam of electrons is bent by a magnetic field. The staff at Sargent Welch helped me determine which power supplies to use to make it work so well.
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