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We are able to affirm that they are present in neutral atoms because the Ray can do
In this video, we are able to see the work of made by the electrons from 2:41 to 2:51. The Crookes' tube contains gas in a low density. When turned on, a ddp - potencial difference - is established between the cathode and the anode. The electrons - negative particles - start flowing from the cathode to the anode, and this movement independent of the gas.