Well I know that ernest rutherford did the gold foil experiment where he fired alpha particles at gold foil. This experiment founded the nucleus but I don't know if the current model of the atom is based on this.
Cathode ray tube experiments sounds like its to do with electrolysis so i dont think it can be that.
The potential difference comes out to be

Given:
σ = 8. 85 × 10-9 c/m2
we know,



given the potential difference between two equipotential surface=5v
E=∆v
∆d=∆v/E


Thus the potential difference is

Learn more about potential difference from here: brainly.com/question/28165869
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So we need to find the formula for magnetic field B using the current (I) and the distance from the probe (d). So, We know that the stronger the current I, the stronger the magnetic field B. That tells us that the I and B are proportional. Also we know that the strength of the magnetic field B is weaker as the distance d of the probe increases. That tells us that B and d are inversely proportional. So our formula should have B=(I/d)*c where c is a constant of proportionality. c=μ₀/2π where μ₀ is the permeability of free space. So finally our formula is B=(μ₀I)/(2πd).