If you need to measure the temperature of something, chances are you can think of half a dozen ways to do it, almost all of which would involve some sort of thermometer, thermistor, thermocouple, or other thermo-adjacent device. But what happens if you need to measure something very hot, hot enough to destroy your instrument? How would you get the job done then?

If you find yourself in this unlikely situation, relax – (Anthony Francis-Jones) has you covered this calorimetric method for measuring high temperatures. The principle is simple; Rather than directly measuring the temperature of the flame, use it to heat something of known mass and composition, then immerse that object in water. If you know the amount of water and its temperature before and after, you can determine the amount of energy contained in the object. From there, you can work backwards and calculate what temperature the object must have been to have that amount of energy.

For the demonstration in the video below, (FJ) suspended a steel ball on a chain in the flame of a Bunsen burner and dipped it in 150 ml of room temperature water. After a long and pleasant toasting, the ball entered the drink, increasing the temperature by 27 degrees. Knowing the specific heat capacity of water and steel as well as the mass of each, he crunched the numbers and came up with an estimate of around 600°C for the flame. It’s way behind; Typical ratings for a natural gas fired burner are in the 1500°C range.

We think the main source of error here is not letting the ball and flame balance, but that doesn’t matter – this is mainly a calorimetry demonstration. This might remind you of bomb calorimetry experiments in a high school physics lab, which can also be used to explore human digestive efficiencyif you like that sort of thing.