Download J. J. Thomson is best known for his discoveries about the nature of

Mass Spectrometer
J. J. Thomson is best known for his discoveries about the nature of cathode rays. His other
important contribution was the invention, together with one of his students, of the mass
spectrometer, a device that measures the ratio of mass m to (positive) charge q of an ion.
The spectrometer consists of two regions as shown in the figure. In the first region an electric
field accelerates the ion and in the second the ion follows a circular arc in a magnetic field. The
radius of curvature of the arc can be measured and then the m/q ratio can be found.
After being accelerated to a speed of 1.51×105 m/s, the particle enters a uniform magnetic field
of strength 0.800 T and travels in a circle of radius 32.0 cm (determined by observing where it
hits the screen as shown in the figure). The results of this experiment allow one to find m/q.
Find the ratio m/q for this particle.
In magnetic field, magnetic force acts in a direction perpendicular to both B and velocity v. As a
result, charge moves in a circle.
mv 2
q
v
1.51105
 qvB  

 5.8984 105 C / Kg . Ans.
Hence,
R
m BR 0.8  0.32