© Randal Oulton
Oven Thermometers have either a broad foot on them, so that they will hook in between the tines of an oven rack and stand up on them without falling over, or a hook to hang from something in the top of your oven. Many have both.
It’s not uncommon for ovens to be off by 50 F. Oven Thermometers are usually not a lot of money: you can get a reliable one for around $10 to $15 / £5.00.
Oven Thermometers can read ranges from 100 to 600 F (38 to 315 C.) You want one with a very large face so that you can read it through the glass in your oven door without having to open the oven door (which of course would cause the temperature to fall.) That’s providing, of course, that your oven door has glass in it — things are coming full circle, and now some of the more expensive ovens don’t have glass in the door (think Agas.)
There are two types of oven thermometers, bimetallic coil and mercury. Mercury are considered more accurate, though you probably really don’t want to break it inside your oven, given how toxic mercury is.
The drag is when you buy 2 or 3 or 4 Oven Thermometers, because you don’t trust the first one, and none of them agree. To see how accurate your oven thermometer is (because the question will no doubt arise in your mind), stick it in a cup of just boiled water — it should read 212 F / 100 C (with adjustments, of course, for your altitude.) If you get a reading that you trust, then if your oven is off, you can calibrate your oven so that the temperatures will match the dial. To calibrate your oven, you adjust the thermostat, but you need to get someone who is qualified to do this, so that you don’t do something that will burn your kitchen down.