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Electron Charge-to-Mass Ratio

In the experiment, observe the trajectory of electron beam in a cathode-ray tube under the effects of accelerating electric field and external magnetic field to measure and calculate the electron charge-mass ratio. In 1897, Joseph Thomson in the Cavendish Laboratory of the University of Cambridge first calculated the charge-mass ratio of particles in the cathode-ray tube with this method, proving that the cathode ray was composed of negatively charged particles much smaller than a hydrogen atom. He named the particle “electron”, which has been used until today.





Connect in series in the circuit. Magnitude of the induced magnetic field can be determined by measuring the current passing through the coil.

Power Supply  
The high voltage (760 ~ 780 V) applied between the
electrodes accelerates electrons to high velocities.
The three rotary controls on the panel are :
Increasing the accelerating voltage increases
         the speed of the electrons.
To focus the electrons into a narrow beam on
         the phosphor screen.
*DC Bias
A bias voltage applied can be used to deflect electrons
        from the center of the phosphor screen.

Output : 0 ~ 50 V AC  power.
The strength of the magnetic field can be controlled by altering the voltage input to the Helmholtz coils.

Helmholtz Coils & Cathode Ray Tube
The homogeneous ac magnetic field is created by Helmholtz coils.
The cathode ray tube accelerates electrons into a low-pressure gas, which glows as the electrons pass through so you can see the path on the phosphor screen.


Electron Charge-to-Mass Ratio


Department of Physics (Digital Archive Project of NTU General Physics Laboratory)
National Taiwan University