OF VITAL PHENOMENA 



39 



a double throw switch, a knife switch (not shown), a telegraph 

 key, k, a potentiometer wire with two (or three) movable con- 

 tacts, a Weston cell and a storage cell. The pieces of apparatus 

 are connected by means of copper wires as shown in Fig. 7. 



The 0.1 n calomel electrode, Fig. 8, is a test tube with syphon 

 side neck, and a platinum wire or foil fused in the bottom. The 

 platinum is plated or covered with pure mercury. Over the mer- 

 cury is placed a layer of calomel (made by precipitation and 

 washed repeatedly with a .1 n solution of KC1). The electrode 

 is filled with the solution of KG and the top closed so that the 

 solution will not run out the syphon. The end of the syphon 

 is closed with some porous material or an ungreased ground glass 

 stopper or stopcock and immersed in a saturated solution of KC1, 

 into which the stopcock of the hydrogen electrode also is dipped. 

 An improvised electrode is shown in Fig. 9 (saturated type). 



Fig. 9. Saturated calomel electrode (from MRR). 



The capillary electrometer may be bought sealed or it may 

 be made in the form shown in Fig. 7. Pure mercury is poured 

 in so as to make contact with the fused in platinum wires in the 

 wide and narrow arms, and about 30 per cent H 2 S0 4 (by weight) 

 is added until the capillary tube is filled. For accurate work 

 this capillary must be fine as thermometer tubing and thin walled, 

 so that higher power lenses may be used. The electrometer is 



