30 
PROFESSOR B. SARDERSOR OR THE ELECTROMOTIVE 
variation was prolonged so as probably to cover a great part of the closing time. In 
the second experiment, the closing time being I/3" later, namely, from 2 , 6" to 3-9" 
after excitation, there was a positive deflection which was not measured. In the third, 
1'3" later, the positive deflection was 90, in the following ones, successively, +71, 
+ 70, +50, +45, +30, +30, and so on. On placing the leaf in the chamber and 
repeating the observation a perfectly similar set of results was obtained. Between 
0'6" and l - 95" after excitation there was a negative deflection of =-380. Between 
1’3 7/ and 2'6" a much smaller negative of —70, these being the expression of the first 
phase. Then followed between 2 - 6" and 3'9" a positive deflection of +6, after which, 
at successive consecutive periods, +100, +75, +70, +42, +20, and so on. 
From these and other similar observations made by the same method, it appears that 
in the fundamental experiment the external surface which, 4" or 5" after excitation, 
attains its greatest positivity as compared with the internal, returns very slowly (if at 
all) to its previous state. Various other methods were used for the purpose of observing 
the progress of the after effect with more completeness, of which the best is exemplified 
in the following experiment. 
The method employed was as follows :—It having been ascertained that the electro¬ 
motive force of the after effect at 2" after excitation corresponded to a deflection of 
160 galvanometer scale, of which the equivalent was 0‘016 D., the galvanometer having 
been over compensated to that extent was closed at 1" after excitation. In this way 
the needle was brought so nearly to rest that it followed the changes of difference 
without oscillation, and could without difficulty be read at 5" intervals. The readings 
were :— 
At 3", at 8", at 1*3", at 18", at 23", at 28" 
+ 9 —50 —130 -142 —149 -160. 
That is, at 3" after excitation the positivity of the external surface was a little more 
than 0’016, at 8" less, at 13" much less, and so on, so that the result in electromotive 
force might be expressed as follows: +0'017 D., +0 # 011 D., +0‘003 D., + 0*002 D., 
+ 0 •001 D. Accordingly, in another leaf, after carefully compensating the existing 
difference and breaking the galvanometer circuit as before, we at once adjusted the 
compensator so that on closing there should be a negative difference of 0*01 D. 
We then excited, and 2" after closed the galvanometer circuit. Again the needle 
followed the slow electrical change, its positions at 5" intervals being as follows : 
+ 70, 0, —60, —120, —140, —150, —150, —160, &c. As the value of 0’01 D. with 
the preparation in circuit was 190 scale, the internal surface was still 30 scale (= about 
0-0015 D.) negative. Repeating the same experiment with an over compensation of 
0’015 D. the successive positions of the image were: +20, —40, —140, —200, —220, 
— 240, —250, — 260, &c. So that the observation indicated, as before, that in this 
instance the negativity of the internal surface exceeded the compensation used. 
In a third leaf observed at the same time, the resistance being unusually low, it was 
