36 ANIMAL ELECTRICITY. — LECTURE 11. 



deflection had fallen from lo to 9 then your deflection 

 20 indicates a pressure of 20 x ^, if it had risen 

 from 10 to 12, then the deflection 20 indicates pres- 

 sure of 20 X j|. 



For nerve, it is convenient to take as a standard 

 the deflection by —^ volt (for muscle we shall take it 

 by ,^„- volt, for the retina by ~^ volt). Say that the 

 pressure of the cell is i volt, this will be done by 

 taking two resistances in the proportion of i and 999 

 (large enough at any rate to let us neglect the internal 

 resistance of the cell), connecting the cell with the two 

 extremes, and the key K^ with the two ends of the 

 smaller resistance. This is because the difference of 

 pressure at the two extremes being i volt, and falling 

 progressively from beginning to end in proportion with 

 resistance, if we take a given resistance between any 

 two points (say i ohm), we shall have a difference of 

 pressure between those two points equal to ^r^g or 

 ^ of the total between the two extremes 



For all our purposes it will be sufficient to take a 

 Leclanche cell (assumed as 1-45 volt) and fixed re- 

 sistances (i and 1450 ohms) to deliver -^ volt. And 

 as a matter of practical convenience, which will be 

 found a convenient ear-mark for reading our plates, 

 we shall put up connections so as always to take this 

 standard deflection in one direction, viz., towards the 

 left or S or negative side of the scale, i.e., downwards 

 on the completed record. 



The usual way of measuring pressure or potential is. 



