A question about the Thomson experiment Recently, I was studying about Thomson's experiment with cathode rays. My textbook shows it like this. 
It says:

When only electric field is applied, the
  electrons deviate from their path and hit the
  cathode ray tube at point A. Similarly when
  only magnetic field is applied, electron strikes
  the cathode ray tube at point C.

But if we apply Fleming's Right Hand Rule, then we get the direction of force in the upward direction, so the rays should deviate towards A but they deviate towards C. I think it is because Fleming's Left Hand Rule is defined for conventional current(flow of positive charges) and what we are dealing with are negative charges. Is that correct?
 A: Fleming's right-hand rule applies when a conductor is moving in a magnetic field and the current is induced.  However, in this case, we have a charged particle moving through a non-varying magnetic field, so it's Lorentz magnetic force law that applies best here.  The simple form, in which there is only a magnetic field component (and no externally applied electrostatic field) is this:
$${\bf F} = q({\bf v} \times {\bf B})$$
in which ${\bf F}$ is the force vector, $q$ is the charge of the moving particle, ${\bf v}$ is the velocity of the particle and ${\bf B}$ is the magnetic field vector.  If we apply the right hand rule to this (the direction of any cross product of vectors can be visualized by using a right hand rule), we see that ${\bf v}$ is to the right when viewing your diagram and ${\bf B}$ is coming out of the page (in the direction of N to S of the magnet) and so the resulting ${\bf F}$ is pointing down.
However, because the charge $q$ is negative, the sign of the resulting vector ${\bf F}$ is reversed, so the force is upward.  The diagram is incorrect and should show reversed N and S poles of the magnet (or equivalently, reversed electrode polarity).
Note, too, that Fleming's left-hand rule (current and magnetic field are given, and force is the result) will also work for direction here because, as you correctly point out, Fleming's left-hand rule is given in terms of "conventional current" (from + to -) rather than electron flow, and electron flow is clearly left-to-right in the diagram.  For the force to be pointing down, again the poles must be reversed.  
Here is a similar diagram figure 2.4 from publisher Prentice-Hall which correctly shows the opposite polarity for the electrodes.
