Here it is. http://www.backyardchickens.com/t/956958/mama-heating-pad-in-the-brooder-picture-heavy-update
You want a heating pad that won't automatically shut off. Several years ago, my heating pad, which was very old, conked out. I bought a new one. After two hours, I noticed it had gone cold. I thought it was defective. I took it back to the store. They told me it was supposed to shut off like that. I was outraged. I said gimme my money back. I'll decide when my heating pad goes off, not some nazi manufacturer.
I bought one that stayed on until I decided to turn it off. Shouldn't humans be in control of their devices? I think so.
Anyway, rant over. You don't want a heating pad that has a mind to quit after two hours on the job. The chicks will be as disappointed as I was, if not more.
And it's not enough just to slip the pad under the straw. You need a wire frame to attach it to so the chicks can crawl under it like they would a broody hen. Pile the straw on top. The whole principle behind the heating pad system is that the chicks crawl under it, their backs touch the warm under-surface of the pad and they absorb heat from being in contact with it.
The beauty of the system is that the rest of the environment can be any temperature, even below freezing, and the chicks will be warm and snug in their cave. They will warm up and then run around outside having a grand time until they need to warm up again. They understand how it works, and learn very quickly when you first place them under it.
After a month or so, the chicks feather out much faster than indoor brooded chicks, and then you can claim the heating pad for your own cold feet or aching back. And you have yourself a pad that will gladly stay on until you tell it that you're done with it.