i think the water behaves as a dielectric at a certain frequency,,, meaning it allows the coil to overcharge the molecules with electrostatic potential...
your osciloscope is analog... may not be bad.. but a digital one is very good option as it is a multimeter inside with multiple functions.. .
diferential probe without proper shield is unreliable.. i wouldnt lose time on it.. try to get the real one is simpler....
i´m using the same tubes as meyer one is a 0,5 inch x 4 inch rod having the holes and being longer than the tube which is 2,75 inch long
its all over... the tubes i got from onlinemetals.com the rods i got and machined here inbrazil
all 304 material seamless tubes ks
in the prototype i´m currently using thereis two sets isolated from eachother and having water in each separated from each other of course... and the electrodes were polished and left over night on citric acid 10% concentration
i believe the cell is a conductor of dielectric field.. .simple as that.. .