I saw parts of National Treasure again recently; I’ve pointed out before that Riley should have used an IR laser to set of the heat sensor in the plan to steal the Declaration. One of the other things about that sequence that has nagged was how fast the thermometer shot up when he zapped it. I can buy that the sensor would trip, since there is a lot less thermal mass, but what about a glass thermometer? The issue is how much thermal mass there is — temperature will respond quickly if there is a small combination of specific heat capacity and mass. I decided to look into this and do a quick experiment.

I grabbed a thermocouple, which I thought would respond fairly quickly: you have a small bead of your dissimilar metals, with a volume of a few mm^3, and since the density of the materials is going to be a little less than 10 mg/mm^3, we’re talking about a few milligrams of material that has a specific heat capacity of around a Joule/gram-Kelvin, so a several Milliwatt laser should be able to raise the temperature in short order. It’s going to depend on how much of the light that hits gets absorbed vs reflected. I have a ~20mW green laser that also emits an unknown amount of IR (the 532 nm is frequency-doubled 1064 nm light, derived from an 808 nm pump, which imperfectly filtered. This can be a safety issue, as explained in this NIST pdf tech note). If we can get 10 mW onto a target with a heat capacity of 10 mJ/K and absorbing 10% of the light, that’s a Kelvin every 10 seconds, or a degree Fahrenheit every 5 seconds.

The response was impressive. In about 30 seconds the indicated temperature jumped almost 7ºF (I used ºF since that’s the scale on the thermometer), which is not as fast as it might have been, but the beam is larger than the target and is well in the ballpark of my prediction and more than enough for what was happening in the movie with a sensor that may have even less thermal mass.

The alcohol thermometer is much more massive. Even though you want to heat up the alcohol, the surrounding glass in contact with it has to heat up as well, so now we’re talking grams of material, so the heating may be slowed by a factor of 100-1000. I shined the laser on the bulb for a full minute and only saw a rise of between 0.5 and 1 ºF. However, confounding this is that the alcohol in my thermometer was without coloring, as opposed to the red I recall in the movie (it was a fairly old device, so maybe it was red at one time, but red dyes have a way of breaking down). Having dye in the alcohol would make it heat up faster. I’m not convinced that it would have risen as far or as fast as was in the movie, but it’s not entirely implausible either.