Physics307L F09:People/DePaula/E/M Ratio

Determining E/M Ratio

Before Millikan constructed his experiment to determine the exact charge of an electron, J. J. Thomson determined the ratio of charge to mass using the following experiment. We attempt to re-create the experiment using a Helmholtz coil, an enclosed electron beam, and a ruler. We hope to obtain a value for E/M that is in accordance with the accepted value of 1.76X10^(-11) coul./kg.

Set Up

I missed the first day this lab was conducted, so set up information will be available at Anne's Page

Results

Using the formulas provided in the Lab Manual, it was easy to calculate the values used to obtain a reasonable e/m ratio. That Excel sheet can be found here Media:E over M data.xlsx

I obtained a value of 3.09E-11 ± 1.48E-11 coul./kg. The current accepted value is 1.76E-11, so my calculated value is off by 44%. This is very disconcerting. I noticed however that the first value obtained for the e/m ratio, in the excel spreadsheet is 1.76E-11 coul./kg. This value is much more legit!