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EXPERIMENTAL GENERAL PHYSIOLOGY 



reservoir R. The reservoir is partly filled with mercury, above 

 which 20 per cent. H 2 SO 4 c. p. fills the reservoir to above the capillary 

 meniscus. Note that platinum wires (w w f ) are fused into the 

 capillary and reservoir passing into the mercury. These wires pass 

 to binding posts and are kept in contact through a short-circuiting 

 key (K). 



The acid must be in contact with the mercury in the capillary. 

 To effect this press the bulb P until the mercury is forced to the 

 tip of the capillary; relieve the pressure and the meniscus will recede 

 drawing the acid after it. Fig. 41 shows how the electrometer is 



FIG. 41 



Showing method of joining up the capillary electrometer E. Note that the positive plate 

 (zinc) is joined through the rheocord -R to the capillary C, while the negative plate (copper) 

 is joined through the rheocord to the reservoir. The battery wires are joined to the zero and 1 

 meter posts of the rheocord, or to the zero and 10 meter posts. In the former case the slider 

 S must be very near, almost touching, the zero post when the first observation of the change 

 of meniscus is made. 



to be joined up for use. Certain precautions should always be 

 observed in the use of the electrometer. The two poles of the instru- 

 ment the mercury in the capillary C and the mercury in the 

 reservoir should be joined through a short-circuiting key, except 

 when one wishes to test difference of electric potential, when the 

 key may be opened for a few moments. The instrument is so sensi- 

 tive that only the weakest currents should be allowed to traverse 

 the acid between the poles. The current from a Daniell cell is 



