ANIMAL ELECTRICITY. — LECTURE II. 35 



the current from T to L, you will find that it will 

 be from S to N through the galvanometer, like- 

 wise that the negative variation from L to T will 

 he from N to S, and when we see the record that 

 1 am about to take during the next half hour, you 

 will realise as convenient and natural that the plate 

 should be turned to show deflections up and down 

 instead of right and left. The negative variation 

 then reads downwards, the current to which it is 

 negative reads upwards. 



I have not yet noticed the apparatus connected 

 with K, for ''Graduation or Compensation," but 

 you will readily appreciate its use if you know that 

 current increases as pressure and inversely as resist- 

 ance. 



Take a case like this : you have made an experi- 

 ment with carbon dioxide, let us say, and on measur- 

 ing out the record you have a negative deflection of 

 10 before and 20 after carbon dioxide. The current 

 has doubled, but this might be by doubled pressure 

 or by halved resistance, or from a mixture of the 

 two changes. You do not know where you are, but 

 you would do so at once if you had as terms of com- 

 parison the deflections made by a convenient standard 

 pressure before and after carbon dioxide. If they 

 were found equal, then the resistance is known to be 

 unaltered, and the augmented deflection was due to 

 pressure only. And from any inequality you would 

 be able to correct for resistance. If the standard 



