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I would like to thank my lab partner Elizabeth Allen, my lab professor Dr.Steven Koch our lab TA Pranav Rathi for their assistance and support in this lab.
I would like to thank my lab partner Elizabeth Allen, my lab professor Dr.Steven Koch our lab TA Pranav Rathi for their assistance and support in this lab.  I would also like to thank my mother(Dixie Colvin) for her help in editing this paper thanks mom.

Revision as of 19:56, 10 December 2009

Experimental Determination of the Electron Charge to Mass Ratio

Author: David Weiss
Experimentalists: David Weiss, Elizabeth Allen
University of New Mexico,
Department of Physics and Astronomy
MSC07 4220, 800 Yale Blvd NE, Albuquerque, New Mexico 87131-0001 USA
Contact info: dweiss01@unm.edu


The ratio for electric charge to the mass of an electron is a fundamental concept in physics and useful for future students interested in the study of physics. From this you can conclude how the electron is hardly affected by gravity and how the electric field governs how the electron behaves. This is important to know for the reason that it is one of the most important values in quantum mechanics. We did this by means of observing the trajectory of electrons in a known constant magnetic field. From this you can find the ratio of electric charge to mass for an electron as a function of observed radii, magnetic field, and energy. This can be done with an electron gun a Helmholtz Coil and a couple of power sources. With all these things we can determine how a beam of electrons curves within a magnetic field and thus measure a radius and with some tricky manipulation figure the ratio for electric charge compared to mass for the electrons. From my experimental data we found that the ratio of e/m is 2.3+/-.23*10 coul/kg. This was one standard deviation away from the accepted value. There was still some systematic and random error that was prevalent throughout the experiment. We will discuss the reasons and sources of these errors.


The charge of an electron is one of the most basic concepts in the entire study of electromagnetism and atomic particles. The first person o find an electron was J.J. Thompson. He did so in a series of experiments which used cathode ray tubes to try to find electrons. He did three such different experiments and it wasn't until the third that he found the charge to mass ratio for the electron which he found in 19872. These results let him to formulate his "Plum Pudding Model" of the atom. This experiment is a lot like the one detailed here. For these experiments he was awarded the Nobel Prize in Physics in the year 1906.

After Thompson did these experiments R.A. Millikan came around and found through experimentation the charge of the electron. His experiments which involve dropping oil droplets in a chamber that could be charged to see how the oil droplets reacted in an electric field. These experiments then lead to the charge that an electron has on it3. He was later awarded the Nobel Prize in Physics for these experiments in 1923 after some controversy due to the deeds of one Felix Ehrenhaft's claim that he found a smaller charge than Millikan, but these claims turned out to be wrong and the prize was given to Millikan.

With out these fundamental experiments we could have not found the charge of the electron, and with out this fundamental constant we could not have been able to do some of the work in chemistry atomic physics and quantum mechanics. The experiment that i did was similar to the experiment that Thompson did in that I am using an electron gun to "boil" off electrons and measure how they behave in a magnetic field. I will vary the force of the electrons by mean of changing the voltage to the electron gun which is the Lorenz Force4, I will also vary the magnetic field by means of changing the current that is applied to the Helmholtz Coils5 to show how an electron responds to a changing electric field and or a changing force.

Experiment and Materials

Instrumentation and Assembly

An electron gun is housed in a bulb that has some gas in to so you can see the electron beam. There is also a Helmholtz Coil attached to this apparatus so that a uniform magnetic field can be generated. This is a manufactured piece so there is no need to worry about aligning everything properly (e/m Experimental Apparatus Model TG-13 Uchida Yoko as shown in Fig. 1). There are three different power supplies each one connects to a different part of the e/m apparatus. A connection needs to be made between the 6-9 Vdc 2A power supply (SOAR corporation DC Power Supply Model 7403, 0-36V, 3A, As shown in Fig.3)and the Helmholtz Coil with a multimeter in series (BK PRECISION Digital Multimeter Model 2831B, Fig 3). The 6.3V power supply (Hewlett-Packard DC Power Supply Model 6384A, Fig. 2)needs to be connected to the heater jacks. A power source rated at 150-300V (Gelman Instrument Company Deluxe Regulated Power Supply, Fig.3) needs to be connected to another multimeter (BK PRECISION Digital Multimeter (Model 2831B, Fig. 2) to the electron gun.

Fig. 1) e/m Experimental Apparatus (Model TG-13),left to right:connections for the Helmholtz Coil power supply,current adjustment knob,focus adjustment,connection to voltmeter for electron gun,connections for the electron gun power supply,conections for the heater power supply
Fig. 2)(bottom) Hewlett-Packard DC Power Supply (Model 6384A, 4-5.5V, 0-8A) connected in series to (top) BK PRECISION Digital Multimeter Model 2831B
Fig. 3) (left of e/m E. A.) SOAR corporation DC Power Supply Model 7403, (top right) BK PRECISION Digital Multimeter Model 2831B, (bottom right) Gelman Instrument Company Deluxe Regulated Power Supply

Procedure and Methods

The general procedure can be found in Professor Gold's Lab Manual1. We first turned on the power to the heater and let it warm up for approximately 2 minutes, we knew this was done when we observed the cathode glowing red. After we warmed up the heater we applied a voltage of 200V to the electron gun, we then observed the beam of electrons that was glowing green. After we observed the electron beam we then applied a current to the Helmholtz Coils, and observed the electron beam take a circular orbit. We then proceeded to take our data on the radii of electron beam, one on the right side and one on the left, in addition to the voltage on the electron gun and the current on the Helmholtz Coils. We took the data on the radii of the beam by looking at a ruler attached to the back of the e/m apparatus(Fig. 1). We noticed how the radii of the beam was effected by changing the voltage while holding the current constant and also with the opposite.

In our experiment we first started holding the current along the coils constant at 1.35A while fluctuating the voltage on the electron gun from a max value of 250V to a minimum voltage of 146V. We observed that the more voltage we applied while keeping the current constant the radius of the electron beam increased. For the next set of experiments we kept the voltage constant at 143V and had a range of current from 0.9A to 1.33A and observed that the radii increased as we decreased the current along the coils. We took data on the radii versus the current and radii versus the voltage and this can be found on my data page for this lab.

Analysis Methods

The theory predicts that there is a relation between the magnetic field and the force at which electrons travel in an electromagnetic field. From this we can use the Boit-Savart Law1 (see Equation 1) to find the magnetic field. The Lorenz Force 4(see Equation 2) can be solved for to find the ratio of e/m by means of knowing the velocity (Equation 3). So knowing this the theory will predict a linear relationship between the Radii and the velocity when current remains constant (Equation 4) with the ratio of m/e being the slope. The theory also predicts that there is also a liner relationship between the radii and the inverse current squared (Equation 5) where voltage remains constant. We used the Microsoft Excel function linest, to find this slope for both the constant current and constant voltage data. [math]Equation 1: B=\frac{\mu R^2NI}{\left(R^2+x^2 \right)^2}[/math]

[math]Equation 2: \vec{F}=e(\vec{v} \times \vec{B}) = m \frac{\vec{v}^{2}}{R}=\frac{e}{m}=\frac{|\vec{v}|}{R|\vec{B}|}[/math]

[math]Equation 3: v=\sqrt{\frac{2eV}{m}}[/math]

[math]Equation 4: \frac{m}{e}\times V=\frac{(7.8\times10^{-4}\times I \times R)^2}{2}[/math]

[math]Equation 5: \frac{m}{e}\times \frac{1}{I^2}=\frac{(7.8\times10 ^{-4}\times R)^2}{2\times V}[/math]

A more detailed method for my calculations can be found here.

Results and Discussion

The data given in Table 1(Fig.4) shows the results that were obtained while keeping the voltage constant at 143 volts. The graph of radii vs 1/I^2, showing how the data compares to a best fit line (using Microsoft Excel,Fig. 5) showing one standard deviation most of the data fits the linear fit. The value of e/v was determined to be 2.74+/-0.38*10^11 coul/kg within 68% confidence interval, which has an error of approximately 55.76% when compared to the value from the paper "The Electronic Atomic Weight and e/m Ratio6" of 1.76*10^11 coul/kg.

From Table 1(Fig. 4) the value of e/m while holding the current constant at 1.35 amps was 1.85+/-0.12*10^11 coul/kg. Using Microsoft Excel a plot of my data radii62 vs voltage(Fig. 6) is given compared to a liner fit of the data, and as there is no obvious systematic deviation this is a good fit for the data. From my value for e/m the error is 5.27% when compared to the vale from "The Electronic Atomic Weight and e/m Ratio6" of 1.76*10^11 coul/kg.

From Table 1(Fig. 4) the value of e/m was calculated while the voltage and current were both being varied. This value was 4.091+/- 0.82*10^11 coul/kg. The liner graph showing the varying values are radii^2 vs voltage(Fig. 7) showin again that even while the voltage and current are varied a linear fit still holds. An error of 76.8% was given when compared to the value in "The Electronic Atomic Weight and e/m Ratio6" of 1.76*10^11 coul/kg. So the data is not good when the current and the voltage are not held constant and an even worse error is generated. {{#widget:Google Spreadsheet |key=0Ao8NF4FsZR3ydHFLU0NSVmdiQ1Y0dVdmclBVVzBKY2c |width=500 |height=300 }}

Fig. 5)Radii versus inverse current(constant voltage 143V).All data falls with in one standard deviations of the liner fit.Made using Microsoft Excel.
Fig. 6)Radii squared vs voltage(constant current 1.35A).All data lies within one standard deviation of the linear fit.Has a better fit than that of the plot of radii vs. current.Made using Microsoft Excel
Fig. 7)Radii squared versus voltage shown with one standard deviation.Most of the data falls along this line but with some outliers so not as good of data.Made using Microsoft Excel


So from my experiment to find the ratio for e/m the best result I found was when I held the voltage constant and varied the radii I came up with a value of [math]1.85\pm.12*10^{11}\frac{coul}{kg}[/math].

So based on my calculations from the above section I obtain the best result when I take the value using constant voltage, which the error is approximately 5.11%. This is not a bad result considering that their is a significant amount of systematic and random error present in this lab. The sources of random(non-biased) error in this lab are as follows. The collisions that occur between the helium atoms in the bulb and discharged electrons from the electrons gun, some of the energy that is used to accelerate the electrons is wasted in the form of visible light due to the collisions so the energy that is measured is an over estimate. Some of the systematic errors are caused by the imprecision of the ruler and in aligning them. We tried to over come some of these by taking two reading one on the left side and one on the right and averaging the two but this can only decrease the error but not eliminate it completely. Another error is that when the voltage is raised or the current is lowered the discharge of the electron beam doesn't line up with where it impacts the electron gun so you can adjust this by means of the focus knob, this changes the radii of the electron beam so you need to adjust the focus every time you change the voltage or current if not you potentially can alter the data and throw off the results.

The experiment showed me that an electron beam when in the presence of a uniform magnetic field creates a circular form. This circle whose radii is directly related to the strength of the magnetic field and the velocity at which the electrons leave the electron gun. If I were to do this experiment again I would not imagine to obtain such a value so close to the actual value for e/m due to the large possibility for errors due to the large amount of systematic errors that can be introduced into the expernment. Summing up even when there is potentially large amounts of errors you can still find fundamental constants.


I would like to thank my lab partner Elizabeth Allen, my lab professor Dr.Steven Koch our lab TA Pranav Rathi for their assistance and support in this lab. I would also like to thank my mother(Dixie Colvin) for her help in editing this paper thanks mom.


1.M. Gold, Physics 307L: Junior Laboratoy, UNM Physics ans Astronomy (2006),[1]

2. J. Thompson, "Cathode Rays". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, Vol 44, 293-316 (1897).

3.R. A. Millikan, "On the elementary electrical charge and the Avogadro constant". The Physical Review, Series II 2: 109–143 (1913).

4.Darrigol, Olivier, "Electrodynamics from Ampère to Einstein",Oxford University Press, ISBN =0-198-50593-0, 327 (2000)[2]

5.R. Merritt, C. Purcell, and G. Stroink. "Uniform magnetic field produced by three, four, and five square coils". Review of scientific Instruments, Volume 54, Issue 7, 879 (1983).

6.R.C. Gibbs and R.C. Williams, "The Electronic Atomic Weight and e/m Ratio". The Physical Review, Volume 44, Issue 12, 1029 (1933).