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A pocket barometer
345 Translated for the Journal of the F r a n k l i n Institute.
2t Pocket Barometer. By M. C. BRtTNS~R, Poggendorff, Annalen, 1854. This instrument is composed of two parts, which are carried separalely, and fitted to each other when the instrument is to be used. One of these pieces, similar to the Forten barometer cistern, is a hollow iron cylinder containing mercury, the level of which can be raised or lowered by the movement of a screw. This reservoir may be closed by a cover when it is to be carried about, and to prevent the mercury from being heated when held in the hand, the cistern is wrapped in woolen stultL The second part is composed of two concentric glass tubes. The external tube, of pretty large diameter s is open below, but hermetically closed at its upper end by a plate of copper traversed by the inner tube, which is open at both ends. To make an observation, the two pieces are put together by screwing. Tile level of the mercury is raised by turning the screw of the cistern, and it is evident that at the moment the lower end of the inner tube dips into the mercury, there is, in the armular space inclosed between the two tubes, a certain volume of air, which will be the same in all observations. The level of the mercury in the cistern is raised until it touches an iron point placed at a suitable height. "[he mass of air is thus reduced to a smaller volume, and in every experiment the diminution ot~ volume is the same. At the same time, by the increase of the elastic force of the interior air, the mercury rises in the central tube, which is open at its upper end, by a quantity which depends on the initial pressure of the atmosphere. To determine exactly the height of the column raised, a rod terminating in a point at its lower end, may be raised or lowered in the inner tube by means of a rack, and a scale fitted to this rod shows the distance of this point, which is made to touch the surface of the mercury in the inner tube, from the point in the cistern, which corresponds to the lower level of the mercury. If h be the height thus indicated, tI the unknown barometric pressure, v and v' the initial and final volumes of the included air, then, if we admit Mariotte's law to be true for air at ordinary pressure, v : v ' :: H q - h : H . This ratio, ~
Vl
V!
WhenceH----h~-Tr.
by which h is to be multiplied in order to get the
V__V ! '
true barometric height, will be constant in all the experiments, and it will suffice to determine this coefficient once for all, by making a comparison with the common barometer. M. Brunner found the value of this coefficient for his apparatus 4"428. To appreciate the value of this instrument, he compared it a number of times with the common barometer ; in the ten comparisons which he has cited, the maximum errors are q- ~'1 millimetres and ~ 0"9 millimetres. He attributes these divergences principally to the changes of temperature which the air may undergo during the observation. On this proposition, the editor of the Bibliotheque Uaiversal~ de Gen&e
346
.Mechanics, Physics, and Chemistry.
remarks, It appears to us that there is a more importan t cause of error which may lead to much more considerable differences; it is the state of moisture of the air. While we may admit Mariotte's law for dry air, or air far from saturation, we certainly cannot do so when it is saturated with watery vapor ; for then the compression will produce condensation of the water. Let us suppose the experiment made under these circumstances ; let us designate, by f , the tension of the vapor of water at saturation, at the temperature at which we are operating, and keep the same vt
notation for the other quantities as before; we shall have H = 7 - ~ - ~ h + f . So that, if we have determined the constant coefficient with air so dry that there was no eondens~ation, and we wished afterwards to make an observation in air saturated at 36 ° Cent., (86 ° Fahr.,) (to take an extreme ease,) we should make an error of nearly 31 millimetres. Even without supposing the saturation complete, it is known that in vessels of glass, Mariotte's law ceases to be true long before the point of saturation, in consequence of the hygroscopic attraction of the glass determining a condensation of vapor. It is therefore probable tha t, if the constant coefficient were determined with a certain degree of dampness, whenever the weather was drier, the pocket barometer would indicate too great, and whenever the weather was damper, too small a height.--Bib. Univ. de GenZve : 21rch. des Sciences Physiques, tome xxvi., p. 162. REmtm~s.~While this reasoning is evidently true, we do not regard the objection as fatal in practice t o lhis very ingenious, and probably very useful instrument; for, by the addition of a chloride of calcium tube~ it appears to us easy enough to fill it always with dryair. ED.
T r a n s l a t e d for tlle J o u r n a l of tile F r a n k l i n Institute.
.3pparatus for preserving Telegraphic ~3pparatus from the disturbing influence of .~tmospheric Electricity. " 1VI. Becquerel presented to the Academy, an apparatus invented by ~I. Barthelemy Bianchi, intended to preserve telegraphic apparatus from the disturbing influence of atmospheric electricity." This very simple and easily erected apparatus, consists of a metallic sphere, traversed by the circuit wire, and kept in the centre of another glass sphere, formed of two hemispheres united by a broad copper ring~ armed at its inside with equi-distant points directed towardsthe centre of the metallic sphere, and approaching within a short distance of its sur~ face. The two hemispheres end in sockets, into which the connecting wire passes and is cemented. The lower part of the copper ring is provided with a metallic stop-cock, which permits a vacuum to be made in the apparatus, and kept in it, if it be thought necessary. This stop-cock has a screw-thread which is to receive a metallic rod designed to put the metallic armature into direct connexion with the earth, while the circuit wire, and the sphere which forms part of it, are Completely insulated. With this apparatus it is seen, that all the atmospheric electricity which
2t Pocket Barometer. By M. C. BRtTNS~R, Poggendorff, Annalen, 1854. This instrument is composed of two parts, which are carried separalely, and fitted to each other when the instrument is to be used. One of these pieces, similar to the Forten barometer cistern, is a hollow iron cylinder containing mercury, the level of which can be raised or lowered by the movement of a screw. This reservoir may be closed by a cover when it is to be carried about, and to prevent the mercury from being heated when held in the hand, the cistern is wrapped in woolen stultL The second part is composed of two concentric glass tubes. The external tube, of pretty large diameter s is open below, but hermetically closed at its upper end by a plate of copper traversed by the inner tube, which is open at both ends. To make an observation, the two pieces are put together by screwing. Tile level of the mercury is raised by turning the screw of the cistern, and it is evident that at the moment the lower end of the inner tube dips into the mercury, there is, in the armular space inclosed between the two tubes, a certain volume of air, which will be the same in all observations. The level of the mercury in the cistern is raised until it touches an iron point placed at a suitable height. "[he mass of air is thus reduced to a smaller volume, and in every experiment the diminution ot~ volume is the same. At the same time, by the increase of the elastic force of the interior air, the mercury rises in the central tube, which is open at its upper end, by a quantity which depends on the initial pressure of the atmosphere. To determine exactly the height of the column raised, a rod terminating in a point at its lower end, may be raised or lowered in the inner tube by means of a rack, and a scale fitted to this rod shows the distance of this point, which is made to touch the surface of the mercury in the inner tube, from the point in the cistern, which corresponds to the lower level of the mercury. If h be the height thus indicated, tI the unknown barometric pressure, v and v' the initial and final volumes of the included air, then, if we admit Mariotte's law to be true for air at ordinary pressure, v : v ' :: H q - h : H . This ratio, ~
Vl
V!
WhenceH----h~-Tr.
by which h is to be multiplied in order to get the
V__V ! '
true barometric height, will be constant in all the experiments, and it will suffice to determine this coefficient once for all, by making a comparison with the common barometer. M. Brunner found the value of this coefficient for his apparatus 4"428. To appreciate the value of this instrument, he compared it a number of times with the common barometer ; in the ten comparisons which he has cited, the maximum errors are q- ~'1 millimetres and ~ 0"9 millimetres. He attributes these divergences principally to the changes of temperature which the air may undergo during the observation. On this proposition, the editor of the Bibliotheque Uaiversal~ de Gen&e
346
.Mechanics, Physics, and Chemistry.
remarks, It appears to us that there is a more importan t cause of error which may lead to much more considerable differences; it is the state of moisture of the air. While we may admit Mariotte's law for dry air, or air far from saturation, we certainly cannot do so when it is saturated with watery vapor ; for then the compression will produce condensation of the water. Let us suppose the experiment made under these circumstances ; let us designate, by f , the tension of the vapor of water at saturation, at the temperature at which we are operating, and keep the same vt
notation for the other quantities as before; we shall have H = 7 - ~ - ~ h + f . So that, if we have determined the constant coefficient with air so dry that there was no eondens~ation, and we wished afterwards to make an observation in air saturated at 36 ° Cent., (86 ° Fahr.,) (to take an extreme ease,) we should make an error of nearly 31 millimetres. Even without supposing the saturation complete, it is known that in vessels of glass, Mariotte's law ceases to be true long before the point of saturation, in consequence of the hygroscopic attraction of the glass determining a condensation of vapor. It is therefore probable tha t, if the constant coefficient were determined with a certain degree of dampness, whenever the weather was drier, the pocket barometer would indicate too great, and whenever the weather was damper, too small a height.--Bib. Univ. de GenZve : 21rch. des Sciences Physiques, tome xxvi., p. 162. REmtm~s.~While this reasoning is evidently true, we do not regard the objection as fatal in practice t o lhis very ingenious, and probably very useful instrument; for, by the addition of a chloride of calcium tube~ it appears to us easy enough to fill it always with dryair. ED.
T r a n s l a t e d for tlle J o u r n a l of tile F r a n k l i n Institute.
.3pparatus for preserving Telegraphic ~3pparatus from the disturbing influence of .~tmospheric Electricity. " 1VI. Becquerel presented to the Academy, an apparatus invented by ~I. Barthelemy Bianchi, intended to preserve telegraphic apparatus from the disturbing influence of atmospheric electricity." This very simple and easily erected apparatus, consists of a metallic sphere, traversed by the circuit wire, and kept in the centre of another glass sphere, formed of two hemispheres united by a broad copper ring~ armed at its inside with equi-distant points directed towardsthe centre of the metallic sphere, and approaching within a short distance of its sur~ face. The two hemispheres end in sockets, into which the connecting wire passes and is cemented. The lower part of the copper ring is provided with a metallic stop-cock, which permits a vacuum to be made in the apparatus, and kept in it, if it be thought necessary. This stop-cock has a screw-thread which is to receive a metallic rod designed to put the metallic armature into direct connexion with the earth, while the circuit wire, and the sphere which forms part of it, are Completely insulated. With this apparatus it is seen, that all the atmospheric electricity which