66 EXPERIMENTAL GENERAL PHYSIOLOGY 



tability in the region of the cathode, and a decreased irritability in 

 the region of the anode. 



Law IV. During electrotonus induced by a strong current the 

 conductivity is decreased in the region of the anode during the 

 passage of the current and in the region of the cathode after removal 

 or breaking of the current. 



These laws account for all typical phenomena observed above. 



XVIII. (a) THE CAPILLARY ELECTROMETER, (b) THE METHOD 

 OF USING IT. 



In those experiments where we have had occasion to measure 

 the strength of an electric current or the difference of potential 

 between two electrodes we have used the tangent galvanometer. 

 But in all these experiments the strength of current or difference of 

 potential has been considerable, amounting in some cases to that 

 represented by several Daniell cells joined in series with a moderate 

 amount of external resistance. 



To detect and measure muscle currents it has been necessary to 

 devise a very delicate and sensitive instrument. The Wiedemann 

 galvanometer has been used for this purpose; but the most simple 

 and satisfactory apparatus is the capillary electrometer. 



(a) The Capillary Electrometer. 



Take a piece of 6-mm. glass tubing and draw two fine capillary 

 tubes; clamp these in burette holders with the capillaries pointing 

 vertically downward. Into one pour a few drops of water; it will 

 pass through the capillary and leave its point drop by drop. Into 

 the second tube pour some mercury enough to fill the capillary 

 and stand 2 cm. or 3 cm. above the capillary in the tube. The mercury 

 will not flow through the capillary. Note that the upper meniscus 

 of the water in the undrawn part of the tube is concave, while 

 the upper meniscus of the mercury is convex. The water wets the 

 glass and seems to be drawn up for a short distance on the vertical 

 surface of the glass, while the mercury does not wet the glass there 

 seems rather to be a repulsion. If one looks at the lower meniscus 

 of the mercury with a low-power microscope, he will find it to be 

 convex downward. 



Mercury stands up in nearly spherical globules on a glass surface,, 

 and water forms nearly spherical globules on an oiled surface. There 

 is no adhesion between the glass and mercury, while there is a strong 

 cohesion between the molecules of the mercury. This accounts for 

 the fact that the mercury forms globules which but for the action 

 of gravitation would be quite spherical. If a drop of liquid be placed 



