THERMO-ELECTRIC NEEDLES. 420 



in the mirror. When the magnet, and with it the mirror, swing out of the mag- 

 netic 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 direc- 

 tion 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 

 moveable 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 



p 



magnetic energy I), I.e., tang. = fy If G is to remain the same, and the tang. < to 



be as large as possible, 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 T) Tm, so that D can be diminished 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 galvan- 

 ometer; (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 sus- 

 pended 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 a current of electricity in this 

 closed circuit, 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 deter- 

 mine 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 1C. aa can be 

 determined by the thermometers. When the current is closed, the excursion on 

 the scale will indicate 1C. Suppose that the excursion was 150 mm., then each 

 mm. of the scale would be equal to 1 |oC. 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 intro- 

 duced 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 



