Excellent post, ke5frf! I think you've said it all.
TY Roff. Don't mean to sound rude myself, but he keeps asking us to not question his intelligence when asking someone if they are using their equipment correctly has no bearing on intelligence whatsoever.
I have a feeling that he IS using his equipment correctly but the diode simply isn't conductiong when he has it set up "correctly",
Going back, he said he refuses to tell us his starting voltages, etc because he doesn't want us to "go off track" with our suggestions. In a way, that is an insult to the person from whom you are seeking advice! In fact, all the voltages and specifics of the circuit very much have a bearing on the outcome. He may not even be applying enough voltage to produce a current through that particular diode for all we know.
In one of his first posts, he said he didn't have time to look up the model number of the diode, which is extremely relevant! Yet he has time to post questions, defend his intelligence, banter, and OUR time is of no consequence.
OK, Mr. Plumber I am sorry for getting testy myself. I truly am. I think you are sincere, but you have to give us ALL pertinent information in order for us to help you, willingly and without hostility.
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EDIT: As an aside comment...if the current measurement with the meter parallel to the diode is not exceeding a milliamp in that configuration, per your observations, a few things you should consider and understand...
First, conceding that I am no expert with electrolysis, it MUST be concluded that the conductivity of the electrolyte solution along with your battery voltage and the resistance of your conductor are the ONLY FACTORS limiting the current in your experiment (in the parallel meter configuration) according to
Ohm's law. This is pure Physics 101 and unless you are the discoverer of a new Earth shattering phenomen this conclusion is all that is left.
What you have is two parallel conductors. IF ANYTHING is certain, your actual current is MORE than your measured current, because the forward biased diode (probably) offers only a little more resistance than the meter and leads, and the overall current will be the sum of the two branches according to Kirchoff's current law. It is entirely conceivable that you have as much as 2 mA of current passing through the circuit but will never know it because to detect it would require two ammeters.
This doesn't seem likely, however, because you say the series configuration isn't working. I would be interested to know how you define not working. In other words, is there NO measured current, or is the current more simply LESS than you expected it to be? If there is no measured current, the only reasonable conclusion is that the battery voltage along with the conductivity of the electrolyte is not enough to properly forward bias the diode and get it to conduct. If there is a current but not what you expect, the only reasonable conclusion is that the overall conductivity of your circuit and the voltage supply is not permitting the current you desire, in part because of the current limiting of the diode.
Are you an actual plumber? OK then you should be familiar with pressure regulators. Think of them for a moment. A 10 lb pressure regulator cannot regulate 5 lbs of water pressure. The supply pressure has to be more than the desired output. You need headroom. This may be the case, in terms of current, with your experiment. By pure coincidence, the electrolyte solution may offer just enough conductivity to permit 1 mA of current with no diode in place (bypassed), but with the diode in place, you simply do not have enough current for it to perform as you desire.
At any rate, without knowing the EXACT performance specifications and conditions of every component in the circuit, there is no way to give you the exact answer you are looking for, only guesses and speculation. But one thing is for certain, your "working" set-up is not working.