62 



EXPERIMENTAL PHYSIOLOGY 



and the battery key (k) open. Bring the mercury meniscus into the 

 field of the microscope. Now open the short-circuiting key. If the 

 electrodes are themselves without current there will be no effect on 

 the electrometer ; but usually there is a slight effect, the direction and 

 the amount of which should be noticed. Next close the battery circuit, 

 leaving the short-circuiting key open. Part of the battery current is 

 now sent through the electrodes and electrometer in a particular 



ejcc 



FIG. 53. Diagram of capillary electrometer. A, Reservoir containing sulphuric 

 acid, s, and mercury, m' ; m, mercury in glass tube drawn out to capillary 

 termination; B, capillary as seen under microscope; n.p., non-polarisable 

 electrodes ; exc, exciting electrodes ; k, k', keys ; c, commutator ; rh, 

 rheochord. 



direction (which can be reversed by the commutator), and there is 

 a corresponding movement of the mercury. Note the direction of this 

 movement, and by following out the wires from the battery determine 

 with which part of the electrometer the anode and kathode are 

 respectively connected. By means of the rheochord and commutator 

 a definite portion of the battery current can be sent in either direction 

 through the electrodes and through any preparation with which they 



