I too am struggling to see why people are having problems with this. I tend to agree completely with Exn.
Somebody posted above somewhere that water is not a conductor... what? Why do you have no ring main in a bathroom, and why is the bathroom light switch operated by a long cord? Of course ordinary tap water conducts! Water might not be the greatest conductor and you won't see much happening at say 12 volts, but it will conduct fine and dandy at mains voltage - especially over here at 240 volts!
As Exn states, this is not electrolysis, the water is simply being heated by the current flowing between the inner and outer bells, we are simply seeing the water being boiled. His fuse blows when he immerses it too far into the water because, as he says, more current flows. Why? Obviously because there is a greater surface area in contact with the water and hence the water resistance is reduced. Pretty elementary stuff really.
Aye, it is rather elementary indeed. When electrically conductive impure water acts as the "resistive heating element" the heating action is focused into the volume of water between the electrodes. It is possible to bring that rather small volume of water to boiling near instantaneously with a power input in excess of 500 Watts. The principle was utilized in years past in small household room humidifiers which heated water to boiling with stainless steel electrodes immersed in a jar of water. A much larger version of the device is used as a dummy load when testing AC Generators; several hundred kilowatts of power will bring a small swimming pool to boiling in a fairly short time. It is perhaps the least costly and most effective kind of dummy load for certain high power applications The full sized commercial versions do control current flow by raising or lowering the electrodes into the conductive water. The similarities are striking.
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