User:Stephen K. Martinez/Notebook/Junior Lab/2008/09/17

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SJK 03:44, 22 October 2008 (EDT)
03:44, 22 October 2008 (EDT)This is a very good lab notebook.  Again, because of the long delay I'm not going to put in a bunch of comments.
03:44, 22 October 2008 (EDT)
This is a very good lab notebook. Again, because of the long delay I'm not going to put in a bunch of comments.


Began with the safety quiz, the main priorities were high voltage and current, essentially by P=VI we needed to determine the most dangerous sources. However, because the voltage sources are grounded and we had no intention of touching the electrodes we assumed for this experiment a caution would be satisfactory to electrical safety. (working with one hand, avoiding 60Hz AC) As far as optical safety, we were informed that the intensity of light would be low enough to neglect, but the x-rays emitted might be damaging as they are ionizing radiation. We settled that this would be a minimum because the manuafacturer would be careful enough, but that we should not expose our delicate eyes to overexhertion. There is a part of this lab where we have to rotate the base of the tube to zero the arc, and it would be necessary to be cautious with the glass, and also as a practice of equipment safety.



  • Soar DC Power Supply Model Number PS-3630 - Hemholtz Supply
  • Soar DC Power Supply Model 7403 UNM 158374 - Heater Supply
  • Gelman Instrument Company Delux Regulated Power Supply - Electrodes
  • Amprobe 37xR-A
  • Amprobe 37xR-A
  • Fluke 111 True RMS Multimeter

We examined our power supplies and found viable cantidates for both the electrodes(40.34mA, and225.1+/-.8V) and Hemholtz (7.495V, and 2.000A), but our third supply was questionble for the heater but it eventually came around through our fidgetting (6.015V, and 1.535+/-.002A). We then hooked these power supplies into the experimental apparatus, making sure to hook up an ammeter in series with the hemholtz coil and power supply and setting the apparatus to e/m measurement. We had one extremeley tempermental multimeter that didn't allow us to measure current, read voltage when disconnected from the power supply, as though discharging a capacitor, and read a signifigantly differant voltage from the other. Therefore our new measurements for the electrodes supply is (42+/-.5mA, 213.4+/-.1V), Hemholtz coil (1.997A, 7.51V), Heater(6.002+/-2V, 1.535A). We also plugged in our good multimeter into the voltage measure on the apparatus.

For Day two of our lab setup we had values:

  • Hemholtz (2.000A, 8.085 +/-0.015V)
  • Electrodes (0.04A, 274.6V)
  • Heater (1.500A, 6.05 +/-.02V)


We are in a dark room and have covered the back of the apparatus with a black hood. We turened on the power supplys and allowed 2min for the filiment to heat up, we had a small problem with our ammeter in series with the hemholtz, but we switched it to parallel and our current recalibrated to apoproximately 2A, so the multimeter must have a circuit inside that adjusts the parallel ciruit to a series. Dr. Koch then informed us that we were stealing all the current from the bending coils by our circuit and we needed to hook it back up the way we had it. Imediatley after we observed the characteristic bending into a circle of the electron beam. The beam was not completely accurate in terminated at the cathode it started at, and portrayed a change in color from initially green/cyan to purple at the top, and in fact with low enough energy the top disappeared altoghether. Dr. Koch helped us theorize that the reason for this could be that the beam loses energy from colliding with the helium atoms (which is necessary for the emission of light we see) and that the two colors might correspond to the two colors associated with the ionization of either one or both electrons throughout the path by progressivley less energetic electrons.

The other theory we considered was that the electric field was signifigantly important to the experiment and the electrons would be decelerated signifigantly after departing the cathode and this caused the loss of energy, then they were reaccelarated as they bent around to approach the cathode again. We then adjusted the accelarating voltage to a level that was enough to change the path to circle and terminate on the back side of the filiment, and then were able to lower that voltage again as we had created a new path through the He for the electron beam to follow. SJK 03:41, 22 October 2008 (EDT)
03:41, 22 October 2008 (EDT) are saying that this hysteresis is evidence of the "path clearing" effect.  I want to think about that more!
03:41, 22 October 2008 (EDT) are saying that this hysteresis is evidence of the "path clearing" effect. I want to think about that more!
There was a signifigant jump at approximately 230 from this passage to path of lowest resistance.

We also adjusted the glass bulb to assure that it terminated on the back side and did not follow a helical path. The cover provided us some trouble as we attempted to adjust the ruler. - suggestion for next year velcro cover. We proceded with the experiment by changing both the voltage (from 230-290V), and the current (1-1.6A) randomly for 10 trials. We followed the lab manual lab manual trying to line up each edge of the circle one at a time with the ruler. As was expected this measurement process was very inaccurate.


mu = 7.8*10^-4 (weber/m^2*amps)

We then created an Excel page to analyze the data, we linearly graphed V vs. r^2 to obtain a best fit line for the slope in the graph of e/m vs. 2/B^2. The slope was off by two orders of magnitude for our average calculated value of e/m so we went back and threw out the values that affected our graph. Our Error from the accepted e/m value of 1.756*10^11C/kg was approximately 70% which is terrible. Some possible reasons for our massive error were disscussed previously: The systematic error for this experiment is massive, the acclearating voltage is not uniform due to the hole in the anode made to view the arc. The electron beam itself is largely affected in the helium cloud it passes through and loses energy to ionize them. The beam is also deccelarated the farther from the potential it travels. The curvature of the glass bulb may have affected our view at larger diameters, and the beam itself had a width. For random error we expect that our eyes would be extremely inaccurate tools to determine the diameter, but we were mostly confident with our electric data recordings. Image:EM.xls
Accelarating Voltage (V) Magnetic Field Current (A) Magnetic Field B = mu*I (Weber/m^2)*10^-4 Diameter 1 (cm) Diameter 2(cm) Average (cm) Error (sum(mean - trials)/2 +/- .3cm line width) Radius(cm)
284.4 +/- .1 1.162 9.063 8 7.5 7.75 0.35 3.875
259.8 1.562 12.184 5 5.5 5.25 0.55 2.625
273.2 1.366 10.655 6.75 6.6 6.675 0.375 3.3375
231.9 1.551 12.098 4.7 4.5 4.6 0.4 2.3
246.4 1.238 9.656 6.25 6.6 6.425 0.475 3.2125
264.0 +/-.1 1.377 10.741 6.3 5.75 6.025 0.575 3.0125
250.7 1.039 8.104 7.25 7.5 7.375 0.425 3.6875
270.6 1.396 10.889 6.5 6.0 6.25 0.55 3.125
240.3 1.598 12.464 4.5 3.7 4.1 0.7 2.05
279.2 1.241 9.680 7.3 7 7.15 0.45 3.575


For the qualitative experiments we first rotated the glass bulb so that the termination of the spiraling electron beam was not on the back end of the cathode. From the Lorentz force we expect the result that under the presence of an electric and magnetic fields an electron will spiral toward the electric force and around the magnetic field lines. F = -q(E + v x B). When we reversed the polarity of the coil leads we found that the electron beam moved downward. We then used the deflector plates that were used in the original experiment to determine e/m. We found that varying the voltage value did not affect the beam deflection as it was indicated to in the lab manual. We could however, tell the polarity of the electric force from the leads and how the beam was affected - namely the beam moved toward the positive plate and away from the negative plate, if we switched the jacks then the plates also switched. When we switched these jacks the beam was deflected downward. Originally the way Thomson set up his apparatus was to correct the deflection of an electric fields influence on the beam with a magnetic field - so we reintroduced our coils. We didn't use this setup because of the inaccuracy of the electric field value but the point at which the magnetic and electric cancelled out was at the values - (V = 250.3 +/- 0.1V and I = 1.750A) and the picture of that is here:


  • Why do we see the electron beam at all?

Because the energy of the electons is transferred to the helium atoms in the tube ionizing them and upon their dissipation of energy to the surrounding they release photons.

  • We ignored Earth's magnetic field in our procedure. How much error does this introduce to the experiment?

Geophysical Website from this site we determined the earths magnetic field to account for approx. 5X 10^-5 T which compared to our values for the measured magnetic field of our apparatus was an order of magnitude off and would not really affect our data.

  • Suppose that protons were emitted in the vacuum tube instead of electrons. How would this effect the experiment?

The deflections would be backwards and we would get a different value of m.

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