Manometer

“ The pressure in the thermometric bulb T is adjusted to equality with the standard by means of a delicate oil-gauge G of small bore, in which the difference of pressure is observed by means of a cathetometer microscope. This kind of gauge permits the rapid observation of small changes of pressure, and is far more accurate and delicate than the mercury manometer. The fundamental measurement of the volume of the overflow in terms of the weight of mercury displaced at 0°C. involves a single weighing made at leisure, and requires no temperature correction. ”

“ It was quite true that the mercury, after rising gradually in the instrument, as long as our descent was taking place, had stopped precisely at twenty-nine inches. "You perceive," said the Professor, "we have as yet only to endure the pressure of air. I am curious to replace the barometer by the manometer." The barometer, in fact, would become useless as soon as the weight of the air was greater than that calculated as about the level of the ocean. ”

“ There are also other instruments which belong to the machine as delicate as it is heavy, and which are used for adjusting the parts of the apparatus, reading the results of a test, or making calculations. This machine differs from others in the way of measuring the force used. It has been the custom to take the pressure on the liquid in the hydraulic cylinder, as shown by a manometer, as the basis of calculation. ”

“ If the pressure is kept constant and equal to the external atmospheric pressure, there is no strain of the bulb, which is particularly important at high temperatures. There is no dead space correction so long as the temperature of the dead space is kept constant. The troublesome operation of reading and adjusting the mercury columns of the manometer is replaced by the simpler and more accurate operation of weighing the mercury displaced, which can be performed at leisure. The uncertain correction for the temperature of the mercury in the manometer is entirely avoided. ”

“ It is therefore necessary to work in a room specially designed to secure great constancy of temperature, and to screen the manometer with the utmost care from the source of heat in measurements of high temperature. Regnault considered that the limit of accuracy of correction was one-tenth of a millimetre of mercury, but it is probably possible to measure to one-hundredth as a mean of several readings under the best conditions, at ordinary temperatures. ”

“ Too hot by half, if the degree of heat was to ascend so high—in which case we should certainly be cooked—not enough, if we wanted to ascertain the exact temperature of springs or metal in a state of fusion.
2. A manometer worked by compressed air, an instrument used to ascertain the upper atmospheric pressure on the level of the ocean. Perhaps a common barometer would not have done as well, the atmospheric pressure being likely to increase in proportion as we descended below the surface of the earth. ”

“ The temperature having returned to its original point, we will give a second stroke of the piston which will rarefy the air more than the former, and thus we will make two observations of the manometer, before and after the return to the former temperature. ”

“ Delicate manometers indicate the pressure at all times, and the most approved electric apparatus is used to indicate the particular point at which a defect has occurred. A skilled engineer is on the watch at all times. Provision is also made so that, in case of any interruption in the regulator, the dials may be run by hand. Accuracy of time is secured by daily comparison with the observatory clock. ”

“ The action of a fly, or vane, suggests the principle of an instrument for measuring the altitude of mountains, which perhaps deserves a trial, since, if it succeed only tolerably, it will form a much more portable instrument than the barometer. It is well known that the barometer indicates the weight of a column of the atmosphere above it, whose base is equal to the bore of the tube. It is also known that the density of the air adjacent to the instrument will depend both on the weight of air above it, and on the heat of the air at that place. ”

“ In many experiments and measurements we know that there is a preponderating tendency to error in one direction. The readings of a thermometer tend to rise as the age of the instrument increases, and no drawing of means will correct this result. Barometers, on the other hand, are likely to read too low instead of too high, owing to the imperfection of the vacuum and the action of capillary attraction. If the mercury be perfectly pure and no appreciable error be due to the measuring apparatus, the best barometer will be that which gives the highest result. ”

“ The close observer gets a much better view-point of daily variations than does the casual observer.
Specific Uses of the Aneroid Barometer.—A custom that is wellnigh universal makes the mercurial barometer the standard instrument for the measurement of pressure at Weather Bureau stations. There is no doubt of the wisdom of this practise. For a substantial instrument not easily getting out of order, and susceptible of close reading, no other form of barometer approaches it. Such instruments as the Marvin normal barograph represent the highest skill in precision instruments. ”

“ The cistern should be from two to three feet above the ground, which will give a height for observing convenient to most persons. A standard barometer should be compared with an observatory standard of acknowledged accuracy, to determine its index error; which, as such instruments are graduated by micrometrical apparatus of great exactitude, will be constant for all parts of the scale. It should be capable of turning on its axis by a movement of the hand, so that little difficulty can ever be experienced in obtaining a good light for observation. ”

“ Using the same barometer a few months afterwards, in the mountains of Jamaica, I was not a little surprised to find that the barometer on one occasion stood higher than it had done when examined three or four hundred feet lower down. Immediately suspecting the cause, and having a fine turnscrew with me, I detached the case altogether from the tube and cistern, and then perceived that a separation had taken place in the column of mercury, low down in the tube, arising doubtless from a small quantity of air which had made its way in, in spite of the usual precautions. ”

“ Pediment Household Barometers, though not so imposing in appearance as the Wheel Barometer, yield direct readings without the intervention of the mechanical appliances necessary for moving a needle over an extended dial. Their mountings are for the most part in oak, walnut, and other woods, the scales are of ivory, porcelain, or enamelled glass, and in their graduation due regard is paid to the relative proportions of cistern and tube, so that the conditions essential to the production of a Standard Barometer are very closely attained. ”

“ When a number of instruments are in use, it is advisable to keep a standard pyrometer for checking purposes, preferably one which has been certified by the National Physical Laboratory. In conducting a test, the couples, with protecting-tubes removed, may be placed in the tube of an electric furnace of the type shown in fig. 29, in close proximity with the standard junction. On raising the temperature gradually, the readings of each working instrument may be compared with the standard, and the necessary corrections discovered. ”

“ Now, with such a statement of errors given in the log for each of the instruments, according to the number, the observations may be properly corrected when they come up here for discussion. It is rare to find a barometer or a thermometer that has no error, as it is to find a chronometer without error. A good thermometer, the error of which the maker should guarantee not to exceed in any part of the scale one degree, will cost, in the United States, not less than $2, perhaps $2.50. ”

“ Nevertheless, the best plan is to dispense with altitude scales, whether fixed or movable, and to calculate the heights. Simple rules, giving more reliable results than the attached scales, are at the service of those who need easy processes. Among these are the following:
Note the rise or fall of the barometer in hundredths of an inch, in passing from one station to the other; multiply by 9. The product is the difference of altitude in feet.
This is for ordinary temperatures and pressures. If the pressure is [Pg 62] below 26 inches or the temperature above 70°, use 10 for a multiplier. ”

“ The value of similar estimates of the rainfall by numerous observers would be very great to meteorology.
116. The principle of measurement in all these gauges is the relation existing between the areas of the collecting and receiving surfaces; that is, between the area of the funnel into which the rain falls, and the area of the cylinder which receives it. In Howard’s and Glaisher’s gauges, this cylinder is virtually the measuring glass itself; in the others, above described, the measuring scales show the same depth of water as in the cylinder of the gauge. ”

“ In the tube anemometer also it is really the pressure that is measured, although the scale is usually graduated as a velocity scale. In cases where the density of the air is not of average value, as on a high mountain, or with an exceptionally low barometer for example, an allowance must be made. Approximately 11/2% should be added to the velocity recorded by a tube anemometer for each 1000 ft. that it stands above sea-level. ”

“ A young and rapidly growing tree is not a desirable object for a station mark; but a mark made on a full-grown tree is not subject to material change. A stone post set firmly in the ground, or a piece of painted scantling attached firmly to the corner of a building will answer the purpose. The mark should be of such a character that it will resist ordinary weathering. The final point in the determination is the station barometer, that is, the chain of determinations which begin at an established plane of reference and end with the ivory point within the cistern of the barometer. ”

“ The reading of a single barometer alone will give us but a faint idea of what is really going to happen from day to day. But if we have a series of barometers located at different stations scattered all over the continent and connected at headquarters by telegraph, so that we can have the readings from a whole series of barometers at once, then it becomes a very useful instrument. A barometer may read low at one station by the scale, but may be high with reference to some other barometer that reads very low. ”

“ High-class instruments, such as that here described, yield exact readings; but, in order to note them accurately, it is important that the eye, the zero edge of the vernier, the top of the mercurial column, and the back of the vernier should be in the same horizontal plane; conditions which may be obtained after some practice.
The accompanying illustration shows a form of barometer which, though not much used in this country, is deservedly popular on the Continent as a standard station barometer. ”

“ This want is now satisfactorily supplied, not by one, but by several descriptions of apparatus. The one first to be described was the design of Admiral Sir A. Milne, who himself constructed, in 1857, we believe, the original instrument, which he used with much success. Since that time several of these instruments have been made, and have performed satisfactorily. The barometer tube is a syphon of large calibre, provided with a Gay Lussac pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as in the “Dial” barometer. ”

“ The engineer who sets out to measure an automobile in all of its capacities and powers must provide himself with tachometers for measuring the engine’s revolutions, dynamometers for testing its tractive force, micrometer calipers for gauging the bore and the stroke, thermometers for measuring its temperature, galvanometers for testing its magneto and battery, and hundreds of 31other instruments, the readings of which must be assembled and studied by means of complex, comparative mathematical formulas before he can tell you what a particular automobile will do. ”

“ After we had done so we would call it an “ammeter” because it measures amperes. Years ago people would have called it an “amperemeter” but no one who is up-to-date would call it so to-day.
If we had a very carefully made ammeter we would send it to the Bureau of Standards to be calibrated. At the Bureau they have a number of meters which they know are correct in their readings. They would put one of their meters and ours into the same circuit so that both carry the same stream of electrons as in Fig. 11. Then whatever the reading was on their meter could be marked opposite the pointer on ours. ”

“ Fig. 27.—Connections for an Installation of Pyrometers.
In some instances a second indicator is kept in the shop office, to which all the pyrometers are wired, and which serves as a standard. The scale of the office indicator is checked daily at one point; and by connecting a given couple first with the shop indicator, and immediately afterwards with the office standard, any errors can be detected. It is also possible to ascertain the temperature of any given furnace in the office at any time, and so to control the whole. ”

“ These figures, with our own tables of wind and flight pressures under all revolution speeds, give us approximately the exact rate of advance. He must watch and keep in operation the barograph or self-recording altitude gauge which takes the place of the usual barometer; as a check he also notes and records periodical readings of the regular barometer; he keeps constant watch on the car equilibrium by means of the statoscope; he records the compass course, sets down the latitude and longitude by following the compass bearing and the advance in miles ”

“ Thus, when by increase of pressure the vacuum box is compressed, the mechanism transfers the movement to the index, and it moves to the right; when the vacuum box bulges out under diminished pressure, the mechanical motion is reversed, and the index moves to the left. As the index traverses the dial, it shows upon the scale the pressure corresponding with that which a good mercurial barometer would at the same time and place indicate ”