did you used horvath circuit? i like those big bubbles, dont know exactly how they are geting that big... perhaps the concave electrode favorites the bubble agregation?
That was just strait dc 3 amps.
I am now going to build that electromagnet and i will build a similar circuit as Horvath is showing in his patents.
Its about 300v and 20 amps being dumped into a coil. He wrote in his patents how many Gauss you should measure in the cell....
If you read the patents, you can see that the electrodes are nickel plated. The hydrogen is collecting some how in that layer.
I think that is part of the reason of those bigger bubles.
Maybe the sharp edges help, but the surface of the inner electrode are also not smooth. Like it was sandblasted.
cheers!