GENEEAL PHYSIOLOGY OF MUSCLE 



71 



Owing to the high resistance of animal tissues (which is millions of times 

 greater than the resistance of metals) and their low potential, it is necessary 

 in electrophysiological research to employ galvanometers or multipliers willi 



FIG. 45. Various forms of unpolarisable electrodes. D and C, du Bois-Reymond 's pattern ; 

 E, Burden-Sanderson's ; B, von Fleischl's ; A, d'Arsonval's. 



many coils and with astatic magnets, so as to render the vibrations as 

 a-periodic as possible. These galvanometers have ahigh internal resistance 

 (5,000-20,000 ohms). The sensitiveness of the instrument can be decreased 

 by a shunt, which cuts off ^, -fifa, or fifrfa 

 of the current. The principle on which gal- 

 vanometers are constructed is that a magnet, 

 suspended and surrounded by a conducting 

 wire, is deflected in the direction of a current 

 passing through the wire, in proportion to 

 the strength of the current. 



Both in Wiedemann's (with detachable 

 and interchangeable spools) and in Thomson's 

 galvanometer (Figs. 43, 44) the deflections of 

 the magnet suspended by a thread of raw silk 

 are more or less magnified by a mirror which 

 reflects a ray of light on to a horizontal scale. 

 These deflections can be photographed on a 

 moving sensitive surface. 



The ends of the galvanometer wires must 

 not be directly applied to the tissues, on 

 account of their polarisability. Unpolarisable 

 electrodes are indispensable in experimenting 

 with muscle and nerve (du Bois-Reymond). 

 These usually consist of a little rod or disc of 

 amalgamated zinc dipping: into solution of FlG - 46. Ostwald's normal electrode, 

 zinc Sulphate in a glass tube, the other end j^ggj ^physiological research 

 of which is closed by a plug of china clay 



saturated with physiological saline, which is in contact with the tissue and 

 protects it from the caustic action of the zinc sulphate (Fig. 45). 



Nowadays, however, all these imperfect electrodes may be replaced by the 

 so-called "normal electrodes" of Ostwald, in which potassium chloride is 



