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 Tesla 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 Tesla 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 Tesla 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

We used the following equipment:

• Soar DC Power Supply Model Number PS-3630 - Power Supply to Coils
• 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