(This is in response to Wednesday’s post, Pin Prick.)
The facebook comments plugin was deactivated this week, but there were a three comments on the facebook post as well. A lot of people who were trying to explain it, but nothing made it clearer than a little demo- my father demonstrated it on a bicycle (no, we did not play a prank- it was our bicycle ;-)). I found this video, which seems like the best demonstration for deflating a tyre:
See the black long rod, that the person is holding in the video? The person is basically pushing the pin in the center. This is what is happening:
Well, this pin is not meant only to deflate the tyre. This pin is more useful, when inflating the tyre. As shown in the upper right image, in its normal position the pin blocks the orifice. But when you put a pump to inflate a tyre, you push the pin down, to open up the orifice for the air to enter. Now the pressure of the air from the pump, is more than the pressure of the air inside the tyre, which is why air enters the tyre (instead of leaving it). Now in the case of the video, or the pranks that children (or as someone in the comment mentioned, even adults play), the pin is pushed down, but the pressure inside the tyre is more than the pressure of the atmosphere outside (in this case, you don’t have a pump pushing air into the hole), which is why the air comes oozing out.
Basically, this pin mechanism is called a valve. A valve is device that allows the flow of fluid in only one direction. Now you might wonder that by definition, the pin mechanism in the tyre should not be called a valve (since it allows air to flow in and out of the tyre), but that pin is there so only to allow air to flow into the tyre. It is a different matter, that we use the device for alternate purposes..
Valves are more ubiquitous than one imagines. There is another very common, intutive valve mechanism:
Other places where a valve is used?
There’s a valve in every heart, and every vain. (Corny touch. I know :-p)