Testing Barometers Hydrometers And Thermometers

In the year 1853 a conference of maritime nations was held at Brussels,

on the subject of meteorology at sea. The report of this conference was

laid before Parliament, and the result was a vote of money for the

purchase of instruments and the discussion of observations, under the

superintendence of the Board of Trade. Arrangements were then made, in

accordance with the views of the Royal Society and the British

ion for the Advancement of Science, for the supply of

instruments properly tested.

In the barometers now in general use by meteorologists on land, the

diameters of the tubes are nearly equal throughout their whole length,

and a provision is made for adjusting the mercury in the cistern to the

zero point, previous to reading the height of the top of the column. The

object of the latter arrangement, it is well known, is to avoid the

necessity of applying a correction to the readings for the difference of

capacity between the cistern and the tube. At sea, barometers of this

construction cannot be used. Part of the tube of the marine barometer

must be very much contracted to prevent "pumping," and the motion of the

ship would render it impracticable to adjust the mercury in the cistern

to the zero point. In the barometer usually employed on shore, the index

error is the same throughout the whole range of scale readings, if the

instrument be properly made; but in nearly all the barometers which have

till recently been employed at sea, the index correction varies through

the range of scale readings, in proportion to the difference of capacity

between the cistern and the tube. To find the index correction for a

land barometer, comparison with a Standard at any part of the scale at

which the mercury may happen to be, is generally considered sufficient.

To test the marine barometer is a work of much more time, since it is

necessary to find the correction for scale readings at about each half

inch throughout the range of atmospheric pressure to which it may be

exposed; and it becomes necessary to have recourse to artificial means

of changing the pressure of the atmosphere on the surface of the mercury

in the cistern.

The barometers intended to be tested are placed, together with a

Standard, in an air-tight chamber, to which an air pump is applied, so

that, by partially exhausting the air, the Standard can be made to read

much lower than the lowest pressure to which marine barometers are

likely to be exposed; and by compressing the air it can be made to read

higher than the mercury ever stands at the level of the sea. The tube of

the Standard is contracted similarly to that of the marine barometer,

but a provision is made for adjusting the mercury in its cistern to the

zero point. Glass windows are inserted in the upper part of the iron

air-chamber, through which the scales of the barometers may be seen; but

as the verniers cannot be moved in the usual way from outside the

chamber, a provision is made for reading the height of the mercury

independent of the verniers attached to the scales of the respective

barometers. At a distance of some five or six feet from the air-tight

chamber a vertical scale is fixed. The divisions on this scale

correspond exactly with those on the tube of the Standard barometer. A

vernier and telescope are made to slide on the scale by means of a rack

and pinion. The telescope has two horizontal wires, one fixed, and the

other moveable by a micrometer, screw so that the difference between

the height of the column of mercury and the nearest division on the

scale of the Standard, and also of all the other barometers placed by

the side of it for comparison, can be measured either with the vertical

scale and vernier or the micrometer wire. The means are thus possessed

of testing barometers for index error in any part of the scale, through

the whole range of atmospheric pressure to which they are likely to be

exposed, and the usual practice is to test them at every half inch from

27.5 to 31 inches.

In this way barometers of various other descriptions have been tested,

and their errors found to be so large that some barometers read half an

inch and upwards too high, while others read as much too low. In some

cases those which were correct in one part of the scale were found to be

from half an inch to an inch wrong in other parts. These barometers were

of the old and ordinary construction. In some the mercury would not

descend lower than about 29 inches, owing to a fault very common in the

construction of the marine barometer till lately in general use, that

the cistern was not large enough to hold the mercury which descended

from the tube in a low atmospheric pressure.

The practice which has long prevailed of mounting the marine barometer

in wood is objectionable. The instrument recently introduced agreeably

to the recommendation of the Kew Committee, is greatly superior to any

other description of marine barometer which has yet been tested, as

regards the accuracy with which it indicates the pressure of the

atmosphere. The diameter of the cistern is about an inch and a quarter,

and that of the tube about a quarter of an inch. The scale, instead of

being divided into inches in the usual way, is shortened in the

proportion of about 0.04 of an inch for every inch. The object of

shortening the scale is to avoid the necessity of applying a correction

for difference of capacity between the cistern and the tube. The

perfection with which this is done may be judged of from the fact, that

of the first twelve barometers tested at the Liverpool Observatory with

an apparatus exactly similar to that used at Kew (whence these

instruments were sent by railway, after being tested and certified), the

index corrections in the two pressures of 28 and 31 inches in three of

them were the same; two differed 0.001 of an inch; and for the remainder

the differences ranged from 0.002 to 0.006 of an inch. The corrections

for capacity were therefore considered perfect, and, with one

unimportant exception, agreed with those given at Kew.

In order to check the pumping of the mercury at sea, the tubes of these

barometers are so contracted, through a few inches, that, when first

suspended, the mercury is perhaps twenty minutes in falling from the top

of the tube to its proper level. When used on shore, this contraction of

the tube causes the marine barometer to be always a little behind an

ordinary barometer, the tube of which is not contracted. The amount

varies according to the rate at which the mercury is rising or falling,

and ranges from 0.00 to 0.02 of an inch. As the motion of the ship at

sea causes the mercury to pass more rapidly through the contracted tube,

the readings are almost the same there as they would be if the tube were

not contracted, and in no case do they differ enough to be of importance

in maritime use.

The method of testing thermometers is so simple as scarcely to require

explanation. For the freezing point, the bulbs and a considerable

portion of the tubes of the thermometers, are immersed in pounded ice.

For the higher temperatures, the thermometers are placed in a

cylindrical glass vessel containing water of the required heat; and the

scales of the thermometers intended to be tested, together with the

Standard with which they are to be compared, are read through the glass.

In this way the scale readings maybe tested at any required degree of

temperature, and the usual practice is to test them at every ten degrees

from 32 deg. to 92 deg. of Fahrenheit. For this range of 60 deg. the makers who

supply Government are limited to 0.6 of a degree as a maximum error of

scale reading; but so accurately are these thermometers made, that it

has not been found necessary to reject more than a very few of them.

* * * * *

Hydrometers are tested by careful immersion in pure distilled water; of

which the specific gravity is taken as unity.

In water less pure, more salt, dense, and buoyant, the instrument floats

higher, carrying more of the graduated scale out of the fluid.

The zero of the scale should be level with the surface of distilled

water, and rise above it in proportion as increase of density causes

less displacement.

The scale is graduated to thousandths--as far as .040 only--because the

sea water usually ranges between 1.014 and about 1.036. Only the last

two figures need be marked.