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تعداد کل یاداشته ها : 11
103/1/31
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مشخصات مدیروبلاگ
 
NA.GH[79]

خبر مایه
بایگانی وبلاگ
 
اسفند 88[1] اردیبهشت 90[5]
لوگوی دوستان
 

پیوند دوستان
 
عاشق آسمونی فانوسهای خاموش جیغ بنفش در ساعت 25 نوری چایی_بیجار مذهب عشق مرجع اس ام اسSMS امپراتوری هخامنشیان ME&YOU شرکت نمین فیلتر دریچه در فراق گل نرگس درختری از جنس باران
عناوین یادداشتهای وبلاگ
Hadrese file injast bardar salam shirin jan 70هستش 69هستش عناوین یادداشتها[11]
69هستش
NA.GH

 

If a particular reading were to represent different degrees of hot or cold depending on location ;thermometers would be useless.
Liquid-in glass thermometers are often used for measuring surface air temperature because they are easy to read and inexpensive to construct.
These thermometers have a glass bulb attached to a sealed;graduated tube about 25 cm (10in).
Long . A very small opening or bore extends from the bulb to the end of the tube.A liquid in the bulb(usually mercury or red-colored alcohol) is free to move from the bulb up through the bore and into the tube.When the air temperature increases the liquid in the bulb expands ;and rises up the tube. When the air temperature decreases the liquid contracts and moves down the tube. Hence the length of the liquid the tube represents the air temperature. Because the bore is very narrow a small temperature chang will show up as a relatively large change in the length of the liquid column.
Maximum and minimum thermometers are liquid –in-glass thermometers used for determining daily maximum and minimum temperatures.The maximum thermomeometer looks like any other liquid-in glass thermometer with one excption:It has a small constriction within the bore just above the bulb (see fig 3.14).
As the air temperature increases the mercury expands and freely moves past the constriction up the tube until the maximum temperature begins to drop the small constriction prevents the mercury from flowing back into the bulb .Thus the end of the stationary mercury column indicates the maximum temperature for the day.
The mercury will stay at this position until either the air warms to a higher reading or the thermometer is reset by whirling it on a special holder and pivot.
Usually the whirling issufficient to push the mercury back in to the bulb past the constriction until the end of the column indicates the present air temperature.*
A minimum thermometer measures the lowest temperature reached during a given period.
Most minimum thermometers use alcohol as a liquid since it freezes at a thermometers
of -130 c compared to -39 c for mercury.
The minimum thermometer is similar to other liquid –in-glass thermometers except that it contains a small barbell-shaped index markerin the bore (see fig 3.15) .The small index marker is free to slide back and forth within the liquid.It cannot move out of the liquid because the surface tension at the end of the liquid column(the meniscus)hols it in.
A minimum thermometer is mounted horizon-tally.
As the air temperature drops the contracting liquid moves back in to the bulb and bring index marker down the bore with it .When the air temperature stops decreasing the liquid and the index marker stop moving down the bore.As the air warms the alcohol expands and moves freely up the tube past the statinary index marker. Because the index marker does not move as the air warms;the minimum temperature is read by observing the upper end of the marker.
To reset a minimum thermometer ;simply tip it upside down.This allows the index marker to slide to the upper end of the alcohol column which is indicating the current air temperature.
The thermometer is then remounted horizontally so that the marker will move toward the bulb as the air temperature decreases. Highly accurate temperature measurements may be made with electrical thermometers such as the thermistor and the electical resistance thermometer.

90/2/4::: 11:43 ع
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