This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
From: David Schwoegler
Tips and Tricks for Drying Foods in Oven
Test the temperature with an instant-read thermometer lying on the middle
oven rack with the thermostat at the lowest setting and watch the reading.
You shouldn't damage the plastic cover on the thermometer at temperatures
Some oven thermostats can maintain 140F using the heating element or burner;
many can't. But don't despair; there's another way. Years ago small metal
"play" ovens were manufactured as toys. The heat source was an electric light
bulb, which baked small cakes and too often burned the young owners. You can
apply the same principal in your gas or electric oven by substituting an
larger electric bulb for the 25W appliance bulb that's already in there.
Fortunately for this purpose, bulbs are rated by their heat output in Watts,
not by their light output in lumens.
Take out the 25W. Turn on the oven light and begin with a 40W, using the
thermometer to monitor the heat gain. Move to larger sizes until you reach
the right sized bulb that gives the temperature you want when it is on
continuously. BE SURE TO REPLACE THE ORIGINAL BULB BEFORE USING THE OVEN
Leaving the door ajar increases the air flow, but also alters the heat loss
characteristics. This is a slow process, but it can work if you are willing
[ the exhaust airflow should blow out a common birthday candle at a distance
of 3-4inches from the exhaust outlet. Increase or decrease heat independently
of air flow. -ED ]