Aberration effect :
Suppose that you are in 1900. You are named Millikan and want to compute the e/m ratio
for the electron.
You will make an experiment with an glass tube without any air in it.
There will be two metal pieces of rectangular shape in it, on which you will put various electrical
A small copper wire, while conducting heavy current, emit electrons through a small hole.
You can control it's speed with an accelerating electrode.
The electron deviate on the effect of the electrical field.
The process implies that you make measures of the electron deviation from the center path.
With a lot of anger the electron does not deviate as much as it must do with classical physics.
If you assume that e is a constant, the term m must have increased.
(Millikan was opposite to Special Relativity and gave out only after 20 years of experimentations.
His measures were finally a strong support for SR)
But you suddenly realized that when the electron enter in the electrical field, the perturbation
must go at nearly the speed of light to the metal piece, then the power source must give energy
to the electron to put it aside, with also a travel time between the metal pieces at
the speed of light.
So the electron is a shorter time in the field (OK it's depend on geometry)
If the time is shorter in the field, deviation is smaller at high speed that at lower.
There is no more use for mass increase, even if the effect is the same.
You imagine Albert Einstein "deconfiture", with a lot of anticipated joy, there is so much time
you are waiting for this event.
You make quick and dirty calculus and found that the effect is the same with good approximation.
You have no doubts that with some refinements you will make your result equal to Lorentz's one