The vacuum appears (and truly becomes) harder, the
fluorescence of the glass shows this, and X-Ray output decreases until the voltage on the coil is increased to keep the ray production constant. Its
especially strange to see an actual puncture in the tube and the same effect... If I didn't witness this with my own eyes I wouldn't believe it. Once
the power is turned off though the tubes fracture or implode. And if the spark gap misfires or the circuit is interrupted in any way you have a
catastrophic failure. This experiment puts you at serious risk in many ways, really not smart.