I made a voltaic pile using 4 pennies, 4 dimes, and paper towels soaked in a salt solution. It had worked perfectly a few days before and I was just confirming that it would work again. Using fresh pennies and dimes, I hooked it up to the multimeter and nothing.
For some reason I took the top dime and towel and turned it over and put it on the bottom. So now instead of d/.../p, I had p/.../d. I switched the leads and presto! 20 mA and a little voltage. I put the dime back on the top and nothing. Back on the bottom and it was good again.
Coins aren't touching, towels are wet, coins have been flipped heads/tails . . . I've tried everything I can think of to find a reason for this to happen. Anyone got any ideas?