46 GENERAL PROPERTIES OF LIVING TISSUES 



The Short-circuiting Key. Two strips of brass, 

 provided with a binding post at each end, are fastened 

 to a block of dark slate (Fig. 9). At the centre of 

 one strip is a post in which is pivoted a copper bar 

 ending in a hard-rubber handle. The bar may be 

 lowered between edges of spring brass. 



Polarization. Connect a platinum and a zinc l 

 plate through a simple key with posts and 20 



Fig. 9. The short-circuiting key ; 

 about three-eighths the actual size. 



Fig. 10. 



of the rheochord as shown in Fig. 10. Connect 

 the zero post and the slider with the capillary 

 electrometer through a short-circuiting key. 



1 It will be observed that the zinc is amalgamated. Chemi- 

 cally pure zinc does not need amalgamation. Commercial zinc 

 contains iron, arsenic, etc., as impurities. The contact of una- 

 malgamated zinc and these dissimilar metals with an electrolyte 

 creates a difference of potential, and parasitic currents run from 

 the zinc to the foreign metals. These currents are prevented by 

 covering the impurities with zinc amalgam, the electromotive 

 properties of which, toward sulphuric acid, are those of pure 

 zinc. As the zinc in the amalgam dissolves out, the film of 

 mercury unites with fresh zinc. Zinc is amalgamated best by 



