METHODS OF ESTIMATING THE TEMPERATURE. 



383 



X 



is first heated to somewhat above 40 C.; then it is copied quickly and at the 

 same time shaken, so that the column of mercury is broken 

 below the upper expansion. Thus the play of the column is 

 from about 40 downward. The tube is so fine that i C. com- 

 prises about 10 cm. in length, and yi^ C. is still i mm. long. 

 A reading of even as little as T ^oo C. has been made possible. 

 The scale is graduated arbitrarily. The value of the graduation 

 must be determined by comparison with a normal thermometer, 

 and also the temperature when the column of mercury reaches a 

 certain level. 



Kronecker and Mayer caused small maximal thermometers 

 to be passed through the digestive canal or through vessels of 

 considerable size. The small instruments are so-called outflow 

 thermometers, whose mercury escapes through the short open 

 tube, and in greater amount naturally when the temperature is 

 highest. After removal, examination is made by comparison with 

 a normal thermometer for the purpose of determining the tem- 

 perature at which the mercury rises exactly to the free extremity 

 of the tube. 



The thermo-electric apparatus permits rapid and accurate 

 measurement of the temperature (Fig. 135, I). The thermo-elec- 

 tro-galvanometer of Meissner and Meyerstein employed for this 

 purpose contains a circular magnet (m) suspended from a silk 

 thread (c) to which by means of a hook a small mirror (s) is 

 attached. Near this magnet another bar-magnet is fixed, with 

 its poles similarly directed, and in such proximity that the free 

 magnet is capable of turning to the north with the slightest de- 

 gree of force. About the latter a thick copper wire (b b) is wound 

 several times (in the diagrammatic representation but one turn 

 is shown), and with the prolonged extremities of this two 

 needle-like thermo-elements (a f , f a) made of different metals 

 German silver and iron and soldered together, are connected. 

 The free ends of these needles of similar name are, further, con- 

 nected by means of a wire (b) . Thus the two thermo-elements are 

 incorporated into the closed circuit. At a distance of three meters 

 from the mirror a horizontal scale (K K) is fixed, the numbers on 

 which are reflected in the mirror. The scale itself is supported 

 upon a telescope (F) , which is directed toward the mirror. The 

 observer (B), looking through the telescope, sees in the mirror 

 the figures of the scale, which can be accurately adjusted. If the 

 magnet swings out of the magnetic meridian, and with it the 

 mirror, other figures on the scale appear to the observer in the 

 mirror. If one of the thermo-elements is heated, an electric cur- 

 rent results, which is directed in the warmer element from the 

 German silver to the iron, and at the same time causes deflection 

 of the movable magnet. If the observer conceive that he is 

 swimming in the direction of the current within the conducting 

 wire the north pole of the magnet is deflected to the left. 



The tangent of the angle, through which the freely movable 

 magnet is deflected from its position of rest in the magnetic me- 

 ridian by means of a galvanic current passed before it, is equal to 

 the relation of the galvanic energy G to the magnetic energy. 

 Therefore, the tangent is as G is to D. In order, thus, to keep 

 the tangent as large as possible, while G remains the same, the 

 magnetic energy must be reduced as much as possible. If the 

 magnetism of the swinging magnet be designated m and the mag- 

 netism of the earth T the magnetic energy D equals Tm. From 

 this it appears that D can be diminished in two ways, namely 

 (i) by reduction of the magnetic force of the swinging magnet, 

 as may be done through the astatic pair of needles of the Nobili 

 multiplicator, and (2) by lessening the magnetism of the earth by 

 means of a fixed auxiliary magnet (Hauy bar) applied in the 

 neighborhood of the swinging magnet with the same object. 



Of importance for the rapid and accurate adjustment of the 

 magnet is the employment of the so-called damping arrangement 

 of Gauss, which is not indicated in the illustration. This consists of a thick, 



FIG. 134 Wal- 

 ferdin's Me- 

 tastatic Ther- 

 mometer. 



