THERMOMETER AND PYROMETER. 



11 



thermometer, but where much accuracy 

 is required there are many niceties that 

 demand attention. 



1. The tube should be of equal 

 diameter throughout the whole stem. 

 As obtained from the glass-house, the 

 tubes are in reality frusta of very elon- 

 gated hollow cones, which, by extension, 

 become more or less nearly cylindrical ; 

 and as the divisions of the scale are 

 usually equal, it is very important that 

 the tube should not perceptibly differ 

 from a true cylinder. 



For these purposes, after a tube has 

 been chosen by the eye as equal in 

 calibre as possible, the best makers 

 blow a bulb on it, and introduce a 

 short column of mercury into the stem, 

 perhaps an inch in length, which is accu- 

 rately measured on a fine scale of equal 

 parts, in different portions of the tube, 

 as the column is, by the heat of the 

 hand, moved from the bulb to the open 

 extremity of the tube. Should the mer- 

 curial column subtend the same 

 number of divisions on the scale in 

 every part of the tube, it may be consi- 

 dered as a perfect tube for a thermome- 

 ter. 



The late Mr. Wilson, of Glasgow, in- 

 troduced thermometric tubes of an 

 elliptical bore. The advantage of this 

 form is, that a very small column of 

 mercuiy is much more visible when it is 

 expanded at right angles to the line of 

 vision. If due precaution be taken to 

 ensure the equality of the tube this 

 form answers well, especially for ordi- 

 nary purposes ; but where great nicety 

 is required, we would recommend the 

 cylindrical tube. 



2. The form and proportion of the 

 bulb may vary according to the purpose 

 for which the instrument is to be ap- 

 plied. The larger the bulb in propor- 

 tion to the stem, so much more deli- 

 cately susceptible of changes of tempe- 

 rature will be the thermometer. The 

 spherical bulb is to be preferred, for 

 this shape is least likely to be affected 

 by the varying pressure of the air ; but 

 when the bulb is very large this form 



"venders the thermometer less susceptible 

 of minute changes of temperature, and 

 pyriform or cylindrical bulbs are usually 

 adopted. All large bulbs are more or less 

 sensibly affected even by slight pres- 

 sure. An examination of more than fifty 

 common thermometers, with large sphe- 

 rical bulbs, in the work- shop of an 

 excellent artist, afforded the writer of 

 this article an opportunity of observing 



that by slightly compressing their bulbs 

 between the finger and thumb, the mer- 

 cury in the stem rose and fell alter- 

 nately several degrees, as the pressure 

 was increased or diminished. The bulb 

 and stem are usually in the same 

 straight line, but for various purposes 

 the bulb is occasionally placed at 

 various angles to the stem. 



In forming the bulb the mouth must 

 not be employed to blow it, otherwise 

 moisture will condense in the tube, 

 which is expelled with much difficulty, 

 and if suffered to remain, will greatly 

 impair the value of the thermometer. 

 Good instrument-makers use a small 

 bottle of caoutchouc, or elastic gum, 

 fastened by a thread on one end of the 

 tube, while the other extremity is soft- 

 ened by the flame of a tallow lamp, 

 urged by a blowpipe. By compressing 

 the bottle, after the orifice of the soft- 

 ened end of the tube is closed by the 

 aid of another rod of glass, a bulb is 

 formed of any required size ; but a neat 

 workman will rarely consider the first 

 blown bulb sufficiently well formed for 

 his purpose. It is generally dilated till 

 it bursts ; the glass, while still soft, is 

 compressed into a rounded mass, and a 

 fresh bulb formed of a regular shape, 

 and size proportioned to the calibre of 

 the tube. Should the artist not intend 

 to fill the tube immediately, he usually 

 hermetically seals the other end of the 

 tube to prevent the entrance of damp 

 air or dust. 



3. The precautions necessary in fill- 

 ing thermometers with mercury are 

 exceedingly well given in Nicholson's 

 Chemistry.* 



The mercury should be clean, dry 

 and recently boiled, to expel air as 

 much as possible. Mercury is often 

 cleaned by thermometer - makers by 

 agitating it in a phial, for some time, 

 with sand, and then straining it through 

 leather ; for nice instruments it should 

 be distilled from iron filings, or reduced 

 from its sulphurets, in clean iron 

 vessels, at a moderate heat. 



The bulb to be filled is heated in the 

 flame of a lamp, and the open extremity 

 of the tube is immersed in the mercury 

 as the bulb cools, the pressure of the 

 atmosphere forces the fluid into the 

 tube and ball. Mr. Nicholson recom- 

 mends, that the bulb should be but 

 moderately heated at first; so as, on 

 cooling, to become only half filled. He 

 advises the open end ot the tube to be 



* Edition 3rd, p. 24. 



