22 GAIiVAKOMETEE. CHAP. ni. 



the binding screws, whilst the other extremities were 

 dijJped into two mercurial cups ; by these means the 

 galvanometer would remain fixed, and it was at these 

 mercurial cups that the contacts were made and 

 broken. The electrodes, by means of which the 

 circuits were formed and completed in the subject 

 during an experiment, consisted of thick platinum 

 wires about nine inches in length. 



There are several circumstances of paramount 

 importance to be attended to during an experiment. 



First. The arrangement of the galvanometer 

 should be perfect, and in a good working condition ; 

 that is to say, the contacts between the different 

 parts should be complete, and no effects should occur 

 upon the needle when a circuit is formed between 

 the mercurial cups : at the same time, the instrument 

 should indicate any slight action from some well- 

 known cause, and if delicate, the mere squeezing of 

 the extremities of the platinum electrodes between 

 the fingers would be quite sufficient to produce an 

 effect upon the needle. 



Secondly. Avoid having any steel about the person, 

 such as pen-knives, metallic buttons, or covered 

 steel buttons in the sleeve of the coat, and when 

 scalpels are used, the experiment should be com- 

 menced on a moveable piece of board away from the 

 galvanometer, and then brought to it. As far as 

 possible, the galvanometer should be out of the 

 influence of iron bars, such as those connected with 

 shutters and fire-irons. 



