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From Wikipedia


A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid to be tested is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a scale inside the stem, so that the specific gravity can be read directly. A variety of scales exist, and are used depending on the context.

Hydrometers may be calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits.


Operation of the hydrometer is based on Archimedes' principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced. Thus, the lower the density of the substance, the farther the hydrometer will sink. (See also Relative density and hydrometers.)


An early description of a hydrometer appears in a letter from Synesius of Cyrene to the Greek scholar Hypatia of Alexandria. In Synesius' fifteenth letter, he requests Hypatia to make a hydrometer for him. Hypatia is given credit for inventing the hydrometer (or hydroscope) sometime in the late 4th century or early 5th century.

The instrument in question is a cylindrical tube, which has the shape of a flute and is about the same size. It has notches in a perpendicular line, by means of which we are able to test the weight of the waters. A cone forms a lid at one of the extremities, closely fitted to the tube. The cone and the tube have one base only. This is called the baryllium. Whenever you place the tube in water, it remains erect. You can then count the notches at your ease, and in this way ascertain the weight of the water.

It was used by Abū Rayh�n al-Bīrūnī in the 11th century and described by Al-Khazini in the 12th century. It later appeared again in the work of Jacques Alexandre César Charles in the 18th century.


In low density liquids such as kerosene, gasoline, and alcohol, the hydrometer will sink deeper, and in high density liquids such as brine, milk, and acids it will not sink so far. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top of the stem, and one for light liquids, on which the mark 1.000 is near the bottom. In many industries a set of hydrometers is used — covering specific gravity ranges of 1.0–0.95, 0.95–0.9 etc. — to provide more precise measurements.


Modern hydrometers usually measure specific gravity but different scales were (and sometimes still are) used in certain industries. Examples include:

Specialized hydrometers

Specialized hydrometers are frequently named for their use: a lactometer, for example, is a hydrometer designed especially for use with dairy products.


A lactometer (or galactometer) is a hydrometer used to test milk. The specific gravity of milk does not give a conclusive indication of its composition since milk contains a variety of substances that are either heavier or lighter than water. Additional tests for fat content are necessary to determine overall composition. The instrument is graduated into a hundred parts. Milk is poured in and allowed to stand until the cream has formed, then the depth of the cream deposit in degrees determines the quality of the milk. Another instrument, invented by Doeffel, is two inches long, divided into 40 parts, beginning at the point to which it sinks when placed in water. Milk unadulterated is shown at 14°.Another method is by using cylindrical vessel made by blowing glass.For calibration it is dipped in pure milk.The point up to which it sinks is marked as "M" .Then the vessel is dipped in water.The level to which it sinks is marked "W".The portion between "M" and "W" is divided into three parts and marked "3" ,"2" ,and "1".When the milk needs to be tested ,it is dipped in the milk to be tested.If it sinks to the"M" mark then the milk is pure.If it sinks up to "3" ,then it's 75% pure ;if it sinks to "2" ,then the milk is 50% pure ;and if it sinks to the "1" mark ,then it is only 25% pure.Milk is denser than water ,therefore it sinks more in water.If the milk is adulterated then ,the lactometer sinks deeper.But this type of lactometer is not very reliable either.It fails to give a correct reading in skimmed milk as it's denser than pure milk.


An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids. It is also known as a proof and traille hydrometer. It only measures the density of the fluid. Certain assumptions are made to estimate the amount of alcohol present in the fluid. Alcoholometers have scales marke

From Yahoo Answers


Answers:Something about whether or not his crown was made out of gold. Then he got inspired in the bathtub when he realized that the weight of water displaced by him was equal to the weight of the part of him that was under the water.

Question:This is a question in my physics textbook... A 3.2-kg balloon is filled with helium (density = 0.179 kg/m^3). If the balloon is a sphere with a radius of 4.9 m, what is the maximum weight it can lift? The answer is supposedly 5.3 kN. This isn't a homework problem or anything - I am just trying to solve it for my own benefit because my professor is a little wacko. Thank you for any help you might have!

Answers:At 20 degrees centigrade and 1 ATM pressure, wikipedia gives the density of air as 1.2 kg/m^3. This means that the volume displaced by the balloon, V = (4/3)*pi*r^3 = 490m^3, amounts to 490 m^3 * 1,2 kg/m^3 = 588 kg of air. The balloon weighs 3,2 kg + 0,179 kg/m^3 * 490 m^3 = 91 kg and thus it can lift 588 kg - 91 kg = 497 kg or 497 kg * 9,81 m/s^2 = 4,88 kN at standard conditions. This is a bit lower than what you were supposed to get, but that might be because of differences in what densities are used and how exact the numbers are.

Question:So what, he got in the bath and the water rose because it was displaced by his body. What's so special about that? I could have told you that when i was 10. If only I had been born in ancient Greece I would have been called a genius. Am I missing something?

Answers:if you can explain the things people like archimedes,newton or einstein without copy the from wikipedia then you will have the right to talk about them.i agree with the one who said that you would be the local idiot

Question:1. WHat was Archimedes doing when he came up with his principle? 2. Why was his principle so revolutionary? 3.What are other improtant events that happend around Archimedes? 4. What does his principle mean?

Answers:Woop. I know these off the top of my head. 1) He was taking a bath. 2) It allowed for the measurement of the volumes/densities of irregularly shaped objects without destroying them. 3) Archimedes invented lots of things, a drill we still use today, in silos to raise grain to the top. Except he used it in a ship. The ship was too big to float. The pressure would have caused the hull to leak. So he said no big deal, let it leak. I can pump it out faster than it can leak in. So they installed the drill into the hull of the boat and created the largest wooden boat to ever float. How does that float your boat? 4) it means that volume and density are proportional to displacement. And for more info... since you didnt read your chapter in your homework... shame shame! The king had a crown made by a goldsmith who was of questionable character. So archimedes was challenged with the task of determining whether the gold was pure or impure, but! He had to do it without melting down the crown. Know the saying, EUREKA!? Yeah, he was so excited that he jumped out of the tub and ran through the streets naked to tell the king, shouting that the whole way. (eureka is greek for "I found it!") He did lots of very cool stuff. He was the master at figuring out "good enough" methods of getting incredible accuracy with incredibly inaccurate tools of the time, relative to our modern technology of course. He was the father of modern calculus as well. He discovered that he could approximate the area of shapes of irregular or circular objects by fitting polygons of increasing numbers of sides to the object, and then approximating. Of course if the mathematics of the time were a little more advanced he would have figured out the underlying function behind the decreasing difference of the areas and gotten the actual result instead of a daringly close approximation.

From Youtube

Archimedes Water Displacement Experiment No.2 :Year 8 travelled to Gordon Primary school to find out about Archimedes.

Archimedes the Greatest Mathematician :Discoveries and inventions : The Golden Crown, The Archimedes Screw, The Claw of Archimedes, The Archimedes Heat Ray...etc Archimedes of Syracuse (Greek: ; c. 287 BC c. 212 BC) was a Greek mathematician, physicist, engineer, inventor, and astronomer. Although few details of his life are known, he is regarded as one of the leading scientists in classical antiquity. Among his advances in physics are the foundations of hydrostatics, statics and the explanation of the principle of the lever. He is credited with designing innovative machines, including siege engines and the screw pump that bears his name. Modern experiments have tested claims that Archimedes designed machines capable of lifting attacking ships out of the water and setting ships on fire using an array of mirrors.[1] Archimedes is generally considered to be the greatest mathematician of antiquity and one of the greatest of all time.[2][3] He used the method of exhaustion to calculate the area under the arc of a parabola with the summation of an infinite series, and gave a remarkably accurate approximation of pi.[4] He also defined the spiral bearing his name, formulas for the volumes of surfaces of revolution and an ingenious system for expressing very large numbers. Archimedes died during the Siege of Syracuse when he was killed by a Roman soldier despite orders that he should not be harmed. Cicero describes visiting the tomb of Archimedes, which was surmounted by a sphere inscribed within a cylinder ...