GOALS

Physics

• Measure the charge-to-mass ratio e/m for electrons of a given kinetic energy moving in a magnetic field.

Technique

• A low pressure Hg gas filled tube and electron accelerator are supplied with radius posts for doing the measurement.
• Helmholtz coils are supplied for producing a uniform magnetic field.
• Try plotting I² (in magnet) vs. l/r² for each V in order to spot systematic errors.

Errors

• This is a good time to evaluate your measurement "standard deviation" because you will have several values of e/m.

for N measurements and x is the mean value of the x_i values.

Question (work out in lab book before lab)

1. Which causes a larger error in your e/m measurement:
   (a) a 5% error in V, the acceleration voltage; or
   (b) a 2% error in x, the separation between the coils of the Helmholtz magnet?

(Note: B = if b=x/2 = a/2)

References

• Bueche, pg. 468; Prob. 16, page 474.