Torricelli later noticed that the height of the mercury varied slightly from day to day, which he correctly attributed to fluctuations in atmospheric pressure. This proved that Aristotle was wrong in believing that a vacuum was impossible. The resulting 3.5 feet of empty space above the mercury in the tube had to be a vacuum, because there was no way for anything else to get in.
(See illustration at right.) Sure enough, the mercury within the tube did NOT remain at a height of six feet, but fell instantly to a level about 2.5 feet above the bowl, in agreement with what Torricelli thought would happen. Torricelli took a six-foot tube filled with mercury and turned it upside down into a bowl of mercury. If the weight of the fluid was all that mattered, then the mercury would rise 13.6 times less than the water, or about 33 / 13.6 = 2.5 feet. Torricelli decided to test his theory by replacing the water with mercury, a heavy (and toxic) liquid metal that is 13.6 times as dense as water.
But if the weight of the atmosphere was less than that of the water, then the air could not lift it high enough to clear the barrier and so the siphon would not flow. Torricelli reasoned that if the weight of Earth's atmosphere was greater than that of the water in the upward-flowing part, then the air could push the water over the barrier and the siphon would flow. Could it be, Torricelli wondered, that a 33-foot-high column of water had exactly the same weight as a column of air extending to the top of the Earth's atmosphere? If so, then what a siphon really represented was a balance - just like a grocery-store balance - except with the weight of the Earth's atmosphere on one side, and the water in the upward-flowing part of the siphon on the other. In 1643 Evangelista Torricelli, who had studied under Galileo, began wondering if the 33-foot limit had more to do with the weight of the air outside the siphon than with any supposed vacuum force inside the siphon. The 33-foot limit represented the maximum force that the vacuum could exert on the water, in Aristotle's view. Sucking the air out of a tube would therefore naturally cause any liquid that might be there to be drawn towards the void, because nature must act to prevent a vacuum. Aristotle had explained this limit by declaring that "nature abhors a vacuum", i.e., Aristotle did not think that a vacuum could exist. A water siphon will only work if the barrier you're going over is less than 33 feet high. Siphons were widely used in Galileo's day to pump water out of mines, for example.īut, there is one major problem with siphoning water. A siphon is a neat way to move a liquid over a barrier with no work and no pumping. As shown at left, a siphon will work even if the fluid first has to flow upwards before it eventually flows down to the bottom. Phyx 103-0, Thermodynamics Dateline Dateline for Thermodynamicsġ643 A siphon is a fluid-filled tube that allows fluids at a higher level to flow to a lower level.
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