THERMOMETERS. ] 



METEOROLOGY. 



1133 



Fig. 30. 



-9 



temperature, is an instrument founded on prin- 

 ciples totally different to 

 any already described. 

 The electrical thermoscope 

 of Nobili admits of being 

 thus described : The ac- 

 tion of the instrument is 

 based upon the funda- 

 mental fact of electro- 

 magnetism ; viz., that a 

 magnetic needle, freely sus- 

 pended and placed in the 

 vicinity of an electric cur- 

 rent, finally arranges itself 

 at right angles to that current. Hence the deflection of a 

 magnetic needle becomes indicative of the existence of 

 such current. Founded on the consideration of this fact, 

 we have the instrument termed the galvanometer, which, 

 in its simplest form, is represented by Fig. 30 ; and a 

 still more delicate construction of which ia represented 

 in the following engraving (Fig. 31). 



Flg.SU 



as employed to indicate small increments of radiant heat 

 evolved from the little lamp on the left, and transmitted 

 through the central diaphragm.* 



It remains now to remark that heat is a fruitful source 

 of electricity, especially when heat is applied to ono end 

 of a mechanical arrangement of alternate bars of two 

 metals, such as bismuth and antimony, as represented in 

 the accompanying diagram (Fig. 32). 



Fig. 32. 



In order to render the combination of metallic bars 

 more compact, they are usually arranged in a bundle, as 

 represented below "(Fig. 33). 



fig. S3. 



Vary few words will now render comprehensible to the 

 'Ins structure and functions of the thermoscope of 

 Nobili. A bundle of bismuth and antimony reduplica- 

 tions, as just described, being placed in communication 

 with a galvanometer, the magnetic needle of the latter 

 is ready to be deflected on the first occurrence of an 

 current, and such electric current is a direct con- 

 s <tu. m;e of the application of heat to one extremity of 

 the system of compound metallic bars. The following 

 diagram, (Fig. 34) represents the thermoscope of Nobili, 



Considerations respecting the Use of Thermometers. 

 Much that is incorrect passes current respecting the 

 functions of the thermometer the accuracy to which it 

 is susceptible, and the spontaneous deteriorations to 

 which the instrument is subject. A few words, 

 therefore, on these matters may be advisable. 



Let us assume the thermometer under con- 

 sideration to be a mercurial thermometer, and 

 that the mercury used was absolutely pure ; that 

 the tube was proved to be of equal bore through- 

 out ; that the process of sealing was accomplished 

 without permitting the ingress of the slightest 

 amount of atmospheric air ; let us, finally, 

 assume that the tube has been accurately 

 graduated such an instrument may be re- 

 garded as free from all errors of construction. 



Nevertheless, it is found that a thermometer 

 thus unexceptionable at first, is liable to de- 

 teriorate by time, the gauge points i.e., 32 

 and 212 corresponding with higher portions 

 of the tube than they should do, and, necessarily, 

 all other degrees similarly change. Most 

 probably this result ia referable to a gradual 

 contraction of the sides of the tube and of the 

 bulb, the contraction being determined by con- 

 tinuous atmospheric pressure. Looking at this 

 contraction, the propriety is suggested of re- 

 taining thermometer tubes filled some time 

 previous to graduation. 



The kind of Heat indicated by Thermometers. The 

 term heat is commonly held to be synonymous with 

 temperature ; but philosophy accepts it in a more ex- 

 tended sense, as comprehending not merely one effect 

 (temperature), but the cause of many effects. The phi- 

 losophy of latent and specific heat Ls almost too purely 

 physical for extended examination here ; hence a slight 

 reference to these conditions will suffice, t The ther- 

 mometer is not adapted to take cognizance of heat in 

 this latent or specific form. It is only indicative of 

 evident heat or temperature ; nor are its indications in 

 this narrow field so complete, nor the information it con- 

 veys so extensive, as is frequently supposed. The ther- 

 mometer does not even profess to indicate the quantity 

 of heat, but only its degree ; terms which are totally dis- 

 tinct, as will soon be perceived. Let us take the follow- 

 ing as an illustration of the difference : A pint of boiling 

 water is as hot as a quart of boiling water, the tempera- 

 ture of both being 212 Fah. ; hence the thermometer, if 

 appealed to, will indicate this identity of temperature. 

 But, necessarily, a quart of boiling water must contain 

 twice as much calorific force as a pint of the same : the 

 deduction is too obvious for comment. 



Again, strictly speaking, the thermometer cannot ba 

 said to present us with the correct temperature of any- 

 thing, inasmuch as the degree of columnar expansion is 

 not the degree corresponding with that of the thing, with 

 which the thermometer is brought into contact, but the 

 mean of the thing touched, and the bulb which touches 

 it. This objection attains its minimum when the atmo- 



For more detailed description* of those instruments, the reader mar 

 consult the .Sections on Electricity and Mat/nrltsin generally, in which 

 these principles of action are investigated. ED. 



t These subjects have been fully discussed in the Section on Beat. 



