THERMO-ELECTKIC NEEDLES. 32 1 



German silver, the two similar free elements being united by a wire (& x ), so that the 

 two thermo-elements form part of a closed circuit. A horizontal scale (K, K) is placed at 

 a distance of 3 metres from the mirror, so that the divisions of the scale are seen in the 

 mirror. The scale itself rests upon a telescope (F) directed towards the mirror. The 

 observer (B), who looks through the telescope, can see the divisions of the scale in the 

 mirror. When the magnet, and with it the mirror, swing out of the magnetic meridian, the 

 observer notices other divisions of the scale in the mirror. When one of the thermo-elements 

 is heated, an electrical current is produced, which passes from the iron to the German silver in 

 the heated couple, and causes a deviation of the suspended magnet. Suppose a person were 

 swimming in the direction of the current in the conducting wire, then the north pole of the 

 magnet goes to the north {Ampere). The tangent of the angle 0, through which the freely 

 movable magnet is diverted by a galvanic current, from its position of rest or zero, in the 

 magnetic meridian, is the same as the galvanic stream; G is proportional to the magnetic 



energy D, i.e., tang. <p = jz. If G is to remain the same, and the tang. <f> to be as large as pos- 

 sible, the magnetic energy must be diminished as much as possible. If the magnetism of the 

 suspended magnet be indicated by m, and that of the earth by T, the magnetic directing energy 

 D = Ti, so that D can be distinguished in two ways : (1) by diminishing the magnetic moment 

 of the suspended magnet, as may be done by using a pair of astatic needles, such as are used in 

 Nobili's galvanometer ; (2) and also by weakening the magnetism of the earth, by placing an 

 accessory stationary magnet (Hauy's rod) in the same direction, and near the suspended magnet. 

 An important arrangement for rapidly getting the magnet to zero is the dead-beat arrangement 

 of Gauss (not figured in the scheme). It consists of a thick copper cylinder, on which the wire 

 of the coil is wound. This mass of copper may be regarded as a closed multiplicator with a 

 very large transverse section. The vibrating magnet induces in this closed circuit a current 

 of electricity, whose intensity is greatest when the velocity of the excursion of the magnet is 

 greatest, and which takes the opposite direction as soon as the magnet returns towards zero. 

 These induced currents cause a diminution of the vibrations of the magnet in this way, that the 

 arc of vibration of the magnet diminishes very rapidly, almost in a geometrical progression. 

 The induced damping-current is stronger, the less the resistance in the closed circuit, and in 

 the damper or dead-beat arrangement itself, the greater the section of the copper ring. This 

 damping arrangement limits the oscillations of the magnet, and it comes to rest rapidly and 

 promptly after 3 or 4 small vibrations, so that much time is saved. The angle of deviation is 

 so small that the angle itself may be taken instead of the tangent. 



The thermo-electric needles of Dutrochet (II) may be placed in the circuit. They consist 

 of iron and German silver soldered at their points ; or the needles of Becquerel (III) may be 

 used. They consist of the same metals soldered in a straight line, one behind the other. The 

 needles must always be covered by a varnish, which will prevent the parenchymatous juices 

 from acting upon them, and so causing a current. Before the experiment we must determine 

 what extent of excursion on the scale is obtained with a certain temperature. In order to 

 determine this, a delicate thermometer is fixed to each of the thermo-couples, and both are 

 placed in oil baths, which differ in temperature say by 1 C. as can be determined by the 

 thermometer. When the current is closed, the excursion on the scale will indicate 1 C. 

 Suppose that the excursion was 150 mm., then each mm. of the scale would be equal to y^ C. 

 When this is determined, the two thermo-needles may be placed in the different tissues or 

 organs of animals, and, of course, we obtain the difference of temperature in these places. 

 Or one thermo-couple may be placed in a bath of constant temperature (nearly that of the 

 body), in which is placed a delicate thermometer, while the other needle is introduced into 

 the organ to be investigated. In this case, we obtain the difference of temperature between 

 the tissue and the source of the constant heat. The electric current passes in the warmer 

 needle from the iron to the German silver, and thus through the wires of the apparatus. 

 For small differences of temperature, such as occur in the body, the thermo-electric energy 

 is always proportional to the difference of temperature of the two needles or couples. In 

 place of a single pair of needles several may be used, whereby the sensitiveness of the apparatus 

 is greatly increased. Helmholtz found that by using sixteen antimony-bismuth couples, he 

 could detect an increase of ^-5-5 Gt Schiffer prepared a simple thermopile (IV) by soldering 

 together alternately four pairs of wires of iron (/) and German silver (a). These are placed 

 in the two organs (A and B) which are to be investigated, whereby a very high degree of 

 exactness is obtained. 



209. TEMPERATURE TOPOGRAPHY. Although the blood, in virtue of 

 its continual motion (completing, as it does, the circulation in twenty-three seconds), 

 must exercise a very considerable influence on the equilibration of the temperature 

 in different organs, nevertheless, a completely uniform temperature does not exist, 

 and the temperature varies in different parts : 



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