V, L, and d are all measurable characteristics of the apparatus If you can measure v x, then you can determine e/m. A magnetic field can be used to just cancel the deflection and determine v x.
A modern application of charge/mass spectroscopy The top quark.
Determining the electron charge e separately. Spray small droplets of oil which quickly reach terminal velocity due to air resistance. Small number of droplets fall between two plates into to a region of constant electric field. Velocity of fall can be estimated by measuring the time to fall a distance d. Ionizing radiation then charges the droplet, introducing an electric force. Charge is quantized. By measuring the velocity of a number of particles with the field on and off and assuming that the electric charges must be multiples of each other, e can be determined.
The discovery of the atomic nucleus: Rutherford Back Scattering what Rutherford expected from the “plum pudding” model the clever experiment the (surprise!) result “It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
When aiming a beam at a thin sample two processes occur: Particles loose energy due to the material’s stopping power There are head on collisions at high angles due to “Rutherford backscattering” scattering depends on mass/size and charge of nucleus Particles loose energy in glancing collisions and interactions with electrons-particles scattered from deep in material are scattered with less energy.
The height of the peak indicates the density of atoms and the ion energy identifies the species and the depth from the surface. High energy edges come from scattering at the material’s surface Ta more likely to scatter Width of Ta peak or width of “Si step” gives thickness of substrate Measuring peak height and normalizing for the scattering spectra for that element give elemental concentrations TaSi film on Si substrate Si TaSi
positive charge is concentrated at the center of the atom in an area ~1/1000 th the size of the atom the mass of the electron is very small compared to the mass of the atom (one thousand times less than the hydrogen atom