You have phases in the alternator, Each coil is wrapped 120 degree's from the other. Each alternator is different.
Alternators have anywhere between 6 to 8 poles on the rotors, Stators vary to matching the rotors. At 3500 rpms with the right pulse train, in my case about 700 to 1000 hertz you get this effect. The frequency depends on the hardware of the alternator. If you have no load after the cell i've recommended then your water could be less conductive, try more in parallel.
The highest voltage you should ever read across any cell is 35 volts, Ideal is around 20 volts, you can also Full wave without the use of 6 diodes, you can full wave with only 2 diodes, This gives you even lower voltages and higher square waves. The setup is not that easy to most, If you have an eye for what to do its a cake walk.
Good luck, It looks like plenty here understands my and stevies setup, assuming you're trying for the waveform i posted.