Physics307L:People/Phillips/Formal Lab Report
Visually Measuring the Charge-to-Mass Ratio for Electrons
Author: Michael R. Phillips
Experimentalists: Michael R. Phillips & Stephen K. Martinez
University of New Mexico: Physics and Astronomy Department, Albuquerque, NM
e-Mail: crooked@unm.edu
Abstract
Using a Helmholtz Coil setup with variable current situated around a Helium filled glass tube with an electron gun at the bottom with a variable accelerating voltage, we measured the diameter of electron beam paths formed into circles by the induced magnetic field from the Helmholtz Coil setup. Using theoretical predictions of the diameter as a function of the charge-to-mass ratio (e/m) for electrons, we were able to form linear data using a least-squares fit and find the slope, which relates to this ratio and some constants. Our final measurement of (4.78 ± 0.041)·1010C/kg was not in very good agreement with the accepted value of 1.759·1011C/kg. The reasons for this large discrepancy will be discussed later.
Introduction
The value calculated during this lab, e/m, seems a strange goal without some inspection of some applications. It is immediately clear, without going through additional examples, that this value is useful because we could actually predict what our diameters would have been in this experiment, given different accelerating voltages and Helmholtz Coil currents, using the same theory we used to find e/m if we only had an accurate value for e/m beforehand. This experiment was first done in a similar fashion that we did by J.J. Thomson (1856-1940) and is described in detail in his 1897 paper.
Methods and Materials
Equipment:
- Soar DC Power Supply Model Number PS-3630 - Power Supply to Helmholtz Coil setup
- Soar DC Power Supply Model 7403 UNM 158374 - Power Supply to Electron Gun Heater Element
- Gelman Instrument Company Delux Regulated Power Supply - Power Supply to Accelerating Electrodes
- 2 x Amprobe 37xR-A - Multimeters
- Fluke 111 True RMS Multimeter
- e/m Apparatus (includes electron gun, Helmholtz coil setup, and Helium-filled tube, and power supply inputs)
- Black Cloth Hood (to block out excess light while taking data)
- Very Many Banana-tipped cords for the Power Supply connections
