I have a Reddy heater that I have been trying to fix. It will come on for a couple of seconds and then shut off so I am assuming it is the photocell. But before I buy the part I want to test this theory by jumping the leads to to the photocell and see if it stays on. My concern is that perhaps the circuit is supposed to have some resistance in it or else it might pass too much current. So are photo cells switches? Or variable resistors?

DESA "says" Measure the resistance across the photocell... should be 11,000 or less. If more, it's defective. Block the photocel with your thumb or tape and look for changes in resistance, if it does NOT change, it's defective. Reddyparts.com