# Measurement of e/m for electrons

## Introduction

In this experiment you will measure $e/m$, the ratio of charge $e$ to mass $m$ of the electron.
The set-up consists of a visible beam cathode ray tube used to produce a beam of
electrons of known energy. The tube contains an electron gun which emits, accelerates,
and focuses electrons. For an electron of charge $e$ and an accelerating voltage $V$ the (non-
relativistic) kinetic energy of the electrons is given by:

$K=\frac{1}{2}mv^{2}=eV$
The tube is supported at the center of a pair of large Helmholtz coils producing a uniform
magnetic field $B$ perpendicular to the velocity of the electrons. Thus, the electrons will be
deflected to trace a circular path with a radius determined by $e/m$, the accelerating voltage
$V$, and the magnetic field $B$ (see equation 1). Helmholtz coils produce a very uniform
magnetic field in the region half-way between the coils defined by:

$B=\frac{\mu_{0}NIR^{2}}{(R^{2}+x^{2})^{3/2}}\,\,\,\,\,\mathrm{(1)}$
where $\mu_{0}=4\pi\times10^{-7} \mathrm{Henry\,m^{-1}}$ is the magnetic permeability of free space, $N = 130$ is the number of turns on each coil, $I$ the coil current, $R$ the mean radius of the coils, and $2x$ thedistance between coils.

## Measurement

Determine $R$ and $x$ (and their errors!) with a meter stick. Then, for a given coil current $I$, vary the accelerating voltage $V$ and read off the radius $r$ of the electron orbit. This is done most accurately by tuning the voltage V such that the edge of the beam coincides with one of the five cross bars in the tube. Perform measurements for as many voltage settings as possible, then repeat the sequence for several different current settings. Be careful NOT TO EXCEED the maximum allowed current of ~2.0A!

## Analysis

The ratio e/m is given by:

$\frac{e}{m}=\frac{2V}{(Br)^{2}}\,\,\,\,\,\mathrm{(2)}$
For each $I$ plot $V$ versus $B^{2}r^{2}$ . You should obtain straight lines. Determine $e/m$ from the slope and average over the different values obtained for different values of $I$. Perform an
error analysis estimating your uncertainties in all measured quantities. Compare your
final result with the literature value and discuss possible deviations. Derive equations (1)
and (2). Discuss how you would improve the precision of the e/m measurement.

## Equipment diagrams