Open circuit voltage

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I did an experiment to try to prove a point to myself or a forum member or a complete stranger 

The point I wanted to make was an open circuit can be constructed and as long as it's open the voltage will remain the same.

Unfortunately I have failed in that endeavor but fortunately succeeded in learning or affirming stuff I have read about.

I plugged in a constant dc power supply 13.8 v and tested voltage 

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Next I grabbed a resistor added it to the open circuit and got some good results 

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So life is good yeah I'm intrigued and fascinated and life is good

But then I got greedy by grabbing a large value resistor and that's when everything fell apart   
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And no I don't understand it but I'm thinking it has something to do with my meter's (internal resistance?)

So yeah I'm off to try and learn about that 

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Ok I have a plan 

I'm gonna go back and increase the resistance in series with (+) and try to discern where voltage begins to drop.

I presume my meter has a 10 megaohm resistance so possibly it's gonna change at the 1 megaohm stage.

 

[g1]

I'm thinking it has something to do with my meter's (internal resistance?) 

Yes.  You could look it up, or you could solve for X: 
8.76/12.80 = x/(x+6)

edit:  my mistake, should have read 13.8, not 12.8, see below.

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Ok I'm back with some results
It dawned on me that I had a decade resistor box so I was like hey that's pretty rad I can just dial in where voltage starts to roll off 

Unfortunately I looked at it and it only goes to 1000k (which I presume is one megaohm) 

Anyhow I started looking around and quickly found a 1M ohm resistor
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Ok cool so let's try 2M ohm
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Ok....there we go

I needed something in between so I looked around and found I think a 220k ohm resistor and added it to the 1M ohm 
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And the multimeter is already taking notice of the increased resistance...I think 

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Yes.  You could look it up, or you could solve for X: 
8.76/12.80 = x/(x+6)
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More homework from G1 

I tried it with the original equation and got a 13 so I looked it up and it said 10 so I retried it using a different denominator and got closer

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