Chap, xi.] TESTING THE ELECTRODES. 115 



to the resistance. It is also to be noted that the con- 

 siderable resistance offered by the deriving vessels is 

 largely counterbalanced by the absence of polarisation 

 which, when properly prepared, they ensure. 



To put the cushions in circuit, carry a fine 

 silk-covered copper wire from the binding screw of 

 one trough to one side of a friction key, and another 

 wire from the second trough to the other side of the 

 key, and then carry a wire from each side of the key 

 to the binding screws of the galvanometer. When 

 the key is closed, the troughs are connected in short 

 circuit ; when the key is opened, they .are placed in 

 communication with the galvanometer. 



The troughs have a CLOSING CUSHION, which is made 

 out of the same blotting paper and saturated with the 

 same solution as the others. It is used for connecting 

 both troughs by being placed as a bridge between the 

 deriving cushions. 



To test the electrodes, connect them with the 

 galvanometer by a friction key as already described ; 

 get the needle of the galvanometer at rest at the zero 

 point in the way already indicated (page 103), connect 

 the two troughs by the closing cushion, and open the 

 key. The needle should remain stationary, indicating 

 absence of all currents from the apparatus. Fre- 

 quently, however, there will be a slight deflection to 

 one side or another, indicating that the two troughs 

 are not quite homogeneous. Close the key, turn the 

 closing cushion so as to change the ends in contact 

 with the troughs, and open the key ; if the deflection 

 is this time to the opposite side, it is this cushion that 

 is at fault. This may be rectified by soaking it fo? 

 some time longer in saturated zinc solution so as to 

 make it homogeneous throughout, or by making a new 

 one. Suppose the changed position of the closing 

 cushion does not alter the deflection, the fault does not 

 rest with the cushion. In the same way, by changing 



