6
e/m Apparatus
012-03471E
!
Measurement of e/m
Notes
1) The greatest source of error in this experiment is the velocity of the electrons. First, the non-uniformity of the
accelerating field caused by the hole in the anode causes the velocity of the electrons to be slightly less than
their theoretical value. Second, collisions with the helium atoms in the tube further rob the electrons of their
velocity. Since the equation for e/m is proportional to 1/r
2
, and r is proportional to v, experimental values for
e/m will be greatly affected by these two effects.
2) To minimize the error due to this lost electron velocity, measure radius to the outside of the beam path.
3) To minimize the relative effect of collisions, keep the accelerating voltage as high as possible. (Above 250V
for best results.) Note, however, that if the voltage is too high, the radius measurement will be distorted by
the curvature of the glass at the edge of the tube. Our best results were made with radii of less than 5cm.
Error bars represent 1mm radius deviation
4) Your experimental values will be higher than theoretical, due to the fact that both major sources of error
cause the radius to be measured as smaller than it should be.
Improving Experimental Results
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1.05
1.1
1.15
1.2
1.25
100
150
200
250
300
350
400
Voltage
1.6A
Best Voltage Range
at this current
