PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
described in Part II. It is to be understood that the electrode[/* is always applied 
to the internal surface of the leaf between tbe three hairs (in the trigone) and the 
electrode m to the outside surface exactly opposite f What happens under this 
condition is always the same. After a delay—of which the duration varies with the 
temperature—the external surface becomes first negative, then positive, to the internal. 
For the purpose of giving a general view of the time-relation of the first phase of the 
variation, I submit the following table, which gives the results of three successive 
rheotome observationst:— 
Time after excitation at which 
galvanometer circuit opened. 
0-2" 
0-4". 
0-6". 
0*8”. 
l"-0. 
l"-2. 
1" '4. 
l"-6. 
Obs. I.—Deflection . 
0 
3 
8 
22 
28 
25 
29 
16 
(100 sc. = 0'063 D.) 
0 
4 
10 
28 
35 
31 
38 
20 
Obs. II. — Deflection . 
1 
19 
30 
45 
44 
29 
8 
2 
(100 sc. = 0-045 D.) 
1 
17 
27 
40 
39 
26 
7 
2 
Obs. III.—Deflection . 
0 
5 
36 
55 
42 
17 
21 
7 
(100 sc.=0-0353 D.) 
0 
3 
25 
39 
30 
13 
15 
5 
Mean result . . . 
0-3 
8 
20 
36 
35 
23 
20 
9 
Each of the leaves employed in the experiments of which the results are given 
was fixed in the glass holder described in Part III., and led off, as above explained, 
by opposite contacts. The three observations were made consecutively; the opposite 
lobe to the one led off 1 was excited by the rheotome (see fig. 6), the electrodes 
used being steel needles, of which the points (2 millims. apart) were inserted into the 
external surface of the lobe. It was consequently necessary to move them three or 
four times during the period of observation, care being taken that each new insertion 
should be at the same distance from the middle line of the leaf as its predecessor. 
Every excitation was repeated twice, and, if the deflections obtained differed in 
extent, a third time. As it was of much more importance to complete the observa¬ 
tion while the leaf remained in a normal condition than to obtain great accuracy in 
* In using the letters / and ^ to designate the leading off electrodes, it is assumed, as in former 
papers, for convenience of description and for no other purpose, that any difference or current is to he 
called negative when it is of such a nature that m becomes negative to /, and vice verso.1. The words 
“ negative ” and “ positive ” therefore have only a relative meaning. 
f The numbers in ordinary figures represent the actual deflection observed ; those in black type, what 
they wmuld have been had the resistance been equal in all the experiments and such that a difference of 
0'05 D. would have been expressed by 100 scale of the galvanometer. The sensibility of the galvano¬ 
meter was purposely reduced for this preliminary experiment, it being desirable to lengthen the periods 
of closure to 0'2 
