A maximum thermometer is set by holding it vertically bulb downwards, and shaking it gently, which lets down the detached portion of the mercury. It is well to compare it with the ordinary thermometer after setting, to be sure that it indicates the temperature of the air. Otherwise, it is possible that the nearness of the observer may have raised its temperature above the following maximum, which would thus be wrongly recorded. Maximum and minimum thermometers are placed nearly horizontal, but they generally mark best when the bulb is placed about half an inch lower than the top of the tube.
4. The minimum thermometer registers the lowest temperature attained since it was last set. Rutherford's minimum is a spirit thermometer containing immersed in the column a glass pin with a small head at each end. Capillary adherence prevents it leaving the column, even when the thermometer is inverted, and the spirit draws it back with it when it contracts, but passes freely round it when it expands. Hence the upper end of the pin (that farthest from bulb) registers the minimum temperature. It is set by being inverted, when the pin descends till it reaches the top of the column.
The makers endeavour to remedy the sluggishness of the spirit thermometer by giving the bulb the shape of a long cylinder, sometimes divided into two, or other shapes that secure a large surface.
5. The solar radiation thermometer is used for registering the relative amount of radiant heat received from the sun. It consists of a maximum thermometer with blackened bulb exposed to the sun's rays. The lamp-black should cover also a portion of the stem, lest the bulb should be cooled by contact with the colder glass. It is generally enclosed in a larger glass cover, from which the air has been exhausted before it was sealed. It is placed on a stand four feet above the ground, the graduated side of the stem up.
The relative amount of solar radiation is indicated by the difference between the temperature registered by this thermometer and that registered by the maximum in shade.
6. The grass minimum thermometer registers the cooling of the surface of the earth owing to radiation towards the sky during the night. It consists of an ordinary minimum thermometer placed on an open grass plain, where it may be protected from injury by a wire cover. The grass must be kept constantly cut short.
The relative amount of radiation from the earth is indicated by the difference between the temperature registered by this thermometer and that registered by the minimum air thermometer. Sometimes it is proposed to secure an extra sensitive instrument to serve as minimum on grass, but it is preferable that the two minimum thermometers should be as like each other as possible.
No self-registering thermometers are used on board ship.
7. It is of importance that the errors of every thermometer used for meteorological observations should be known within a tenth of a degree at all temperatures. This is required even in case of a station where the thermometers are read to the nearest whole degree, for an error of a tenth of a degree would still vitiate the monthly mean temperature. The errors should be determined at a central observatory by careful comparison with its standard thermometer.
The Hongkong Observatory undertakes to furnish corrections to be applied to thermometers sent there by observers who regularly send their registers to the Government Astronomer. It is particularly necessary to have black bulb thermometers verified by comparison with a standard in a central observatory, as the readings are largely affected by the thickness of the lamp-black and the degree of vacuum in the glass.
The glass of the bulb undergoes a contraction for years after it is made, and it is therefore found that a thermometer after some time reads too high even when the corrections are properly applied. The observer may then have to re-determine the correction at some temperature. It is generally recommended to verify the freezing point by immersing the thermometer in crushed ice melting in a warm room. Care should be taken to immerse not only the bulb but also as great a part of the stem as possible, at least up to within a few degrees of the freezing point, and the thermometer should remain for some time in the melting ice.
Thermometers should be verified in the same position in which they are to be read. The correction of a thermometer that is placed horizontally can be determined by laying it down flat in a large basin of water, and comparing it with a thermometer whose corrections are known, also immersed in the water. The bulbs should nearly touch each other, and the water should be agitated. It must not be forgotten that one thermometer may be more sluggish than another.
A comparison of thermometers placed side by side in the open air is generally of little use.
8. A thermometer is liable, when carried about, to have its column broken. This does not generally happen to a mercurial thermometer, unless a minute speck of air has got into the mercury. It is often repaired by holding the thermometer, which should be gently heated, vertically in the hand and knocking the hand (not the thermometer) against the table. The air-speck is thus pushed higher up in the column, and when the bulb is subsequently allowed to rest in very cold water, it clings to the side of the tube. The operation is to be repeated until the column has passed below the air-speck, which then joins the air in the upper part of the tube, from which air is seldom completely absent. In fact, it is an advantage in case of the thermometer that the vacuum should not be perfect.
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A maximum thermometer is set by holding it vertically bulb downwards, and shaking it gently, which lets down the detached portion of the mercury. It is well to compare it with the ordinary thermometer after setting, to be sure that it indicates the temperature of the air. Otherwise it is possible that the nearness of the observer may have raised its temperature above the following maximum, which would thus be wrongly recorded. Maximum and minimum thermometers are placed nearly horizontal, but they generally mark best when the bulb is placed about half an inch lower than the top of the tube.
4. The minimum thermometer registers the lowest temperature attained since it was last set. Rutherford's minimum is a spirit thermometer containing immersed in the columen a glass pin with a small head at each end. Capillary adherence prevents it leaving the column, even when the thermometer is inverted, and the spirit draws it back with it when it contracts, but passes freely round it when it expands. Hence the upper end of the pin (that farthest from bulb) registers the minimum temperature. It is set by being inverted, when the pin descends till it reaches the top of the column.
The makers endeavour to remedy the sluggishness of the spirit thermometer by giving the bulb the shape of a long cylinder, sometimes divided into two, or other shapes that secure a large surface.
5. The solar radiation thermometer is used for registering the relative amount of radiant heat received from the sun. It consists of a maximum thermometer with blackened bulb exposed to the sun's rays. The lamp-black should cover also a portion of the stem, lest the bulb should be cooled by contact with the colder glass. It is generally enclosed in a larger glass cover, from which the air has been exhausted before it was sealed. It is placed on a stand four feet above the ground, the graduated side of the stem up.
The relative amount of solar radiation is indicated by the difference between the temperature registered by this thermometer and that registered by the maximum in shade.
6. The grass minimum thermometer registers the cooling of the surface of the earth owing to radiation towards the sky during the night. It consists of an ordinary minimum thermometer placed on an open grass plain, where it may be protected from injury by a wire cover. The grass must be kept constantly cut short.
The relative amount of radiation from the earth is indicated by the difference between the temperature registered by this thermometer and that registered by the minimum air thermometer. Sometimes it is proposed to secure an extra sensitive instrument to serve as minimum on grass, but it is preferable that the two minimum thermometers should be as like each other as possible.
No self-registering thermometers are used on board ship.
7. It is of importance that the errors of every thermometer used for meteorological observations should be known within a tenth of a degree at all temperatures. This is required even in case of a station where the thermometers are read to the nearest whole degree, for an error of a tenth of a degree would still vitiate the monthly mean temperature. The errors should be determined at a central observatory by careful comparison with its standard thermometer.
The Hongkong Observatory undertakes to furnish corrections to be applied to thermometers sent there by observers who regularly send their registers to the Government Astronomer. It is particularly necessary to have black bulb thermometers verified by comparison with a standard in a central observatory, as the readings are largely affected by the thickness of the lamp-black and the degree of vacuum in the glass.
The glass of the bulb undergoes a contraction for years after it is made, and it is therefore found that a thermometer after some time reads too high even when the corrections are properly applied. The observer may then have to re-de- termine the correction at some temperature. It is generally recommended to verify the freezing point by immersing the thermometer in crushed ice melting in a warm room. Care should be taken to immerse not only the bulb but also as great a part of the stem as possible, at least up to within a few degrees of the freezing point, and the thermometer should remain for some time in the melting ice.
Thermometers should be verified in the same position in which they are to be read. The correction of a thermometer that is placed horizontally can be determined by laying it down flat in a large basin of water, and comparing it with a thermometer whose corrections are known, also immersed in the water. The bulbs should nearly touch each other, and the water should be agitated. It must not be forgotten that one thermometer may be more sluggish than another.
A comparison of thermometers placed side by side in the open air is generally of little use.
8. A thermometer is liable, when carried about, to have its column broken. This does not generally happen to a mercurial thermometer, nnless a minute speck of air has got into the mercury. It is often repaired by holding the ther- mometer, which should be gently heated, vertically in the hand and knocking the hand (not the thermometer) against the table. The air-speck is thus pushed higher up in the column, and when the bulb is subsequently allowed to rest in very cold water it clings to the side of the tube. The operation is to be repeated until the column has passed below the air-speck, which then joins the air in the upper part of the tube, from which air is seldom completely absent. In fact it is an advantage in case of the thermometer that the vacuum should not be perfect.
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