when there is a fire, the sprinklers installed on the roof of the building can start spraying water automatically. how do you do this? The Slow Mo Guys recently photographed the start-up of a common sprinkler with a high-speed camera.

(source: The Slow Mo Guys)

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the sprinkler usually turns on automatically when the local temperature rises to a certain extent. The device that triggers the water spray can use a low melting point alloy or a heat-sensitive glass tube like the one pictured above. According to what I have seen, the glass tube is sealed with a solution of alcohol or glycerol, and when heated to a certain extent, the vaporization of the liquid and the expansion of the liquid itself will produce enough pressure to burst the glass tube. This glass tube usually acts as a "plug" against a piece of metal so that the water will not flow out, so when it breaks, the valve will open and the water will spray out of the sprinkler. If you look carefully, you can see from the picture above that a piece of metal fell off after losing the support of the glass tube, and then a lot of water was spurted out.

(the sprinkler looks like this when it is not started. Image source: Wikipedia)

the liquid inside the glass tube here is red, this is a specially pigmented color, which is used to indicate the different response temperature of the product, as shown in the following picture:

(picture source: Wikipedia)

where you need a spray device, you can choose the glass tube that starts at different temperatures according to your needs, which can be kept sensitive while avoiding the loss caused by spraying water easily.

I have seen sprinklers in some buildings before, but I have never observed them carefully. Next time I see them, be sure to climb up and see if there is a small glass tube. (for the original video of

TheSlow Mo Guys, see: https://www.youtube.com/watch?v=NXAgjKDFc14