Bloody Electrostatics demonstrations

Got up at 6:30 this morning to be in school at 7:15 to have lots of time to prepare for a form 5 class on electrostatics which I wanted to film.
Now I'm not normally this dedicated, but because it was being videod ('videoed'?) I wanted to get everything right.
Electrostatics is dodgy at the best of times, but at half eight this morning every thing was going like a dream. To such an extent that I started wondering why other teachers made such a big deal of it. Maybe they should prepare more - like I was doing.

Class began at about 12 o clock. I started the video, and spent the first ten minutes correcting homework on the board, then started into the demos.
Not one worked as well as it did in the morning.
Some didn't work at all.
It was baffling, frustrating and funny in roughly equal measures.

I take that back.
Mostly it was frustrating, especially since I had prepared it so well.
The smug factor had felt good too.

It's possible that atmospheric conditions had changed over a few hours, but I suspect one other variable was that the classroom had had four sets of students sit in there for forty minutes at a time, each breathing in nice clean dry air, and breathing out air which contained a higher percentage of water. As one of the students said: "Come on now sir, I know you blame us for everything else, but you can't seriously think you can blame us for this one."
So;
tomorrow I repeat the process first thing in the morning, then again a few hours later, and if the same thing happens I think I might use hot plates or bunsen burners to dry out the air for a spell and then repeat (what happens the water / water vapour when the air drys? Where does it go?).
Edge of the seat stuff this . . .