Cathode rays travelled in straight lines and if an opaque object was placed in their path, a shadow of that object appeared. This seemed to suggest cathode rays were waves, but charged particles can also travel in straight lines and be stopped by an object. Particle supporters suggested that the rays had momentum and mass since they turned small paddlewheels placed in the path of rays. However, wave advocators said that a radiometer also turned under light. Remaining evidence for them being waves is that they can pass through thin sheets of metal foil without damaging them. And for particles, evidence is that they were obviously deflected by magnetic fields and they travelled considerably more slowly than light. The main restriction for the charged particle theory was the absence of deflection in electric fields as shown by Hertz. But Thomson found that within the tube, cathode rays ionised the gas, and ions were attracted to the plate with opposite charge. So the ions neutralised the charge on the plate, allowing the cathode rays to pass by unaffected - (a). Plates had to be sealed within the tube, and after evacuating air from the tube, Thomson used a tube with more complete vacuum than Hertz, and observed deflection of cathode rays due to an electric field, towards the positive plate - (b). This proved beyond doubt that cathode rays were not electromagnetic radiation but streams of negatively charged particles (electrons). But the ability for cathode rays to penetrate thin metal foil without damaging them was still unexplained due to inadequate knowledge of the nature of atoms.