Describe an experiment to measure the ratio of the charge to mass of an electron

Describe an experiment to measure the ratio of the charge to mass of an electron

• The electrons are produced thermionically by a hot filament cathode and are accelerated towards a cylindrical anode and pass through it.
• The small hole on the anode confines the electrons to a narrow beam.
• When both the electric field and the magnetic field are off, the electrons reach the screen at X and cause fluorescence.
• If the velocity of the electrons on emerging from the anode is u then

• The magnetic field is switched on and the beam is deflected to position Y.
• In order to bring the beam back to the original position X, the electric field is switched on and adjusted until the beam is at X again.
• This implies that The magnetic force = the electric force

Beu = eE

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