Oven Thermometers Not Just Eye Candy

Here's a picture (click the pic) of me turning setting the oven to preheat while I made my Cherry and Golden Raisin Scones. You can see that I've set the temperature for 425 degrees. On my whirlpool oven, the preheat timer starts at 5 minutes and 15 seconds and then beeps at me when it is preheated. For years ... probably two or three years, TheWife and I wondered why some of our food just didn't turn out exactly right. Sometimes she would bake cakes or cookies and the cook time would be 4-6 minutes longer than the recipe said. For cookies, that's HUGE! So here's what we found:

Like my Ove' Glove?

As you can see, my supposedly 425 degree preheated oven barely showed a temperature of 200 degrees. Upon learning this information years back, TheWife and I breathed a collective "WTF?" So you just can't trust an oven's preheat thermometer? I don't know. But I would suggest picking up an oven thermometer. When you are baking, steady temperature is critical, the temp can't be rising or falling when a cake is in the oven. Chemistry gets confused that way.