This question has been closed: https://physics.stackexchange.com/questions/688998/how-was-mass-increase-measuredMeasurement because:

I’m voting to close this question because it concerns the history of physics – Bob D

and then:

Closed. This question is off-topic. It is not currently accepting answers.

I have been around for a few years and have never heard anything so weird.

There are loads of questions bordering history, but mine does not:

• Who (and Why) started the "electrons are negative, protons are positive" convention?

And many more concerning past experiments:

• How to measure the mass of the electron?. I have chosen two examples answered by an illustrious member, and I hope there was no discrimination.

As I clearly explained, the question was absolutely legitimate, since in 1894 the mass spectrometer was not yet available and Thomson had not even discovered the electron. Is that really off-topic?