placed vertically, the mercury falls, until it reaches the height (about 30 inches at sea level) at which its pressure on its base is equal to that exerted by the atmosphere on an equal area of the mercury in the cistern, and subsequently it rises and falls with increasing and diminishing atmospheric pressure.
2. The height of the mercury in the tube is measured with a carefully divided brass rod, which for the sake of convenience is permanently attached to the barometer beside the tube. The lower end of the rod, which is generally tipped with ivory, having been adjusted to touch the surface of the mercury in the cistern, and one or two taps with the finger having been given to the top of the barometer to agitate the mercury in the column, in order to get rid of any capillary adherence, a pointer is adjusted on a level with the top of the convex surface of the mercury in the tube, and the height of the barometer is then read on the scale of the rod.
A piece of white paper should be pasted behind the top of the tube and also behind the cistern so as to reflect light towards the observer. The front edge of the pointer, the top of the mercury, and the back edge of the pointer should be exactly on a level. When pointing it is necessary to keep the eye exactly on the same level, and after pointing the observer should raise and lower his head a few inches, to be sure that there is no bright space visible between the lower edges of the pointer and the mercury.
In order to facilitate the accurate reading of the height of the column a moveable scale, termed the vernier, is attached to the pointer. The principle of this contrivance is, that the total number of smallest subdivisions on the vernier is made equal to one more, or less than that number in a length of the fixed scale equal to the length of the vernier.
The long lines cut on the barometer scale correspond to tenths of an inch, and the short lines to five-hundredths, while every long line cut on the vernier corresponds to hundredths, and every short line to two-thousandths of an inch.
The following diagrams clearly exhibit the mode of reading the barometer. In fig. 1, the edge of the vernier (the pointer) intersects the scale above the division 29.80 and below 29.85. Write down 29.800 as the scale reading. Then, running the eye up the vernier, the second short line after third long line is seen to coincide with a scale division. Its value 0.030 + 0.004 added to 29.800 gives 29.834 as the reading.
M
31
31
30-
-30-
X
29
Fig. 1.
29
Fig. 2.
I
placed vertically, the mercury falls, until it reaches the height (about 30 inches at sea level) at which its pressure on its base is equal to that exerted by the atmosphere on an equal area of the mercury in the cistern, and subsequently it rises and falls with increasing and diminishing atmospheric pressure.
2. The height of the mercury in the tube is measured with a carefully divided brass rod, which for the sake of convenience is permanently attached to the barometer beside the tube. The lower end of the rod, which is generally tipped with ivory, having been adjusted to touch the surface of the mercury in the cistern, and one or two taps with the finger having been given to the top of the barometer to agitate the mercury in the column, in order to get rid of any capillary adherence, a pointer is adjusted on a level with the top of the convex surface of the mercury in the tube, and the height of the barometer is then read on the scale of the rod.
A piece of white paper should be pasted behind the top of the tube and also behind the cistern so as to reflect light towards the observer. The front edge of the pointer, the top of the mercury, and the back edge of the pointer should be. exactly on a level. When pointing it is necessary to keep the eye exactly on the same level, and after pointing the observer should raise and lower his head a few inches, to be sure that there is no bright space visible between the lower edges of the pointer and the mercury.
In order to facilitate the accurate reading of the height of the column a moveable scale, termed the vernier, is attached to the pointer. The principle of this contrivance is, that the total number of smallest subdivisions on the vernier is made equal to one more, or less than that number in a length of the fixed scale equal to the length of the vernier.
The long lines cut on the barometer scale correspond to tenths of an inch, and the short lines to five-hundredths, while every long line cut on the vernier corresponds to hundredths, and every short line to two-thousandths of an inch.
The following diagrams clearly exhibit the mode of reading the barometer. In fig. 1, the edge of the vernier (the pointer) intersects the scale above the division 29.80 and below 29.85. Write down 29,800 as the scale reading. Then, running the eye up the vernier, the second short line after third long line is seen to coincide with a scale division. Its value 0.030 + 0.004 added to 29.800 gives 29.834 as the reading.
M
31
31
30-
-30-
X
29
Fig. 1.
29
Fig. 2.
I
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