Chadwick's experiment and proton velocity

[Ferrus]

How did he measue the proton velocity in the polonium-beryllium experiment? The only source with some description describes his using current as a means to do so - but sure that would require knowledge of the number of protons in order to ascertain the drift velocity?

 

[cragar]

I Read a little about the experiment . He may have accelerated the protons through a voltage. K=qV k= kinetic energy q=charge V=voltage . And then he could have deflected the particles in a magnetic field
using the Lorentz force . F=qvB*sin(x)
Not sure about the Lorentz force but accelerating them through a voltage would be reasonable.
Not sure though .