MUSCLE 



579 



On the former principle the thermopile has been constructed 

 (Fig. 182), on the latter the bolometer, or < electrical-resistance ther- 

 mometer' 



Where no very fine differences of temperature are to be measured, 

 a single thermo-junction of German silver and iron, or copper and iron, 

 is inserted into ; a muscle or between two muscles. But the electro- 

 motive force, and therefore the strength of the thermo-electric 

 current, is proportional for any given pair of metals to the number of 

 jurictions, and for delicate measurements it may be necessary to use 

 several connected together m series. A thermopile of antimony- 

 bismuth junctions gives a stronger current for a given difference of 

 temperature than the 

 same number of German 

 silver-iron 'couples, but 

 from its brittle nature is 

 otherwise less convenient. 



The direction of the 

 current in the circuit is 

 sucnthatit passes through 

 the heated junction from 

 bismuth to antimony, and 

 from copper or German 

 silver to iron. Knowing 

 this direction, , we are 

 aware of the changes of 

 temperature which take 

 place from the movements 

 of the mirror of the gal- 



FIG. 182. 



A, a single copper-iron thermo-electric couple ; 

 B, two pairs, one inserted into the tissue b, the 

 other dipping into water in a beaker a. The tem- 

 perature of the water may be adjusted so that the 

 galvanometer shows no deflection. The temperature 

 of the tissue is then the same as that of the water. 



vanorneter with which the 

 pile is connected. The 

 galvanometer must be of 

 low resistance, since the 

 electromotive force of 

 the thermo-electric cur- 

 rents is small, and a high resistance would cut down their intensity 

 too much. 



The muscle which is to be excited is brought into close 

 contact with one junction or set of junctions, the other set 

 being kept at constant temperature by immersing them in 

 water, or covering them with muscle that is not to be 

 stimulated. The image will now come to rest on the scale ; 

 and excitation of the muscle will cause a movement indicat- 

 ing an increase of temperature in it, the amount of which 

 can be calculated from the deflection. 



In this way Helmholtz observed a rise of temperature of 



