28 
PROFESSOR B. SANDERSON ON THE ELECTROMOTIVE 
It having been first ascertained that when K 2 was at 0*2" of the scale of the 
pendulum, and K 3 at 0'04" there was no variation, successive observations were 
taken with K 3 at positions further and further removed from K 2 (which remained 
stationary at 0*02") with the following results :—With K 3 at (FOG" there was a trace 
of movement of the needle; with K 3 at 0*08" it had increased to —4 scale ; at OT" 
to 12 ; at 0*15" to —39 sc. To complete the observation K 2 was moved to 0*1" 
and K 3 to 0*2", so that the period of closure now corresponded to the second tenth 
of a second after excitation; the galvanometer reading was —161 sc.; finally, 
between 0*2" and 0*3" we had —248 sc. 
In four other experiments the results (stated shortly) were as follows :— 
Experiment I. Temperature 32-35° C. 0*02 // -0*4", no deflection; 0*02 // -0*05 // , 
a trace ; 0*02"-0*06", —5 sc.; 0*02 // -0*l // , —35 sc., —31 sc. 
Experiment II. Temperature 34° C. 0*02"-0 , 04 // , no deflection; 0*02 // -0*05' / , 
-3 sc. ; 0’02 // -0’06 // , -6 sc. ; 0*02"-0T5", —70 sc. 
Experiment III. Temperature 32° C. 0 *02 // -0*06" no deflection; 0*02 // -0*08 // , no 
deflection ; 0*02 // -0*10", —13 sc., 22 sc., 23 sc. ; 0'02"-0*9", —10 sc., 7 sc., after 
which previous negative results were verified by repetition. 
Experiment IV. Temperature 36° C. 0*02 // -0*04", trace ; 0*02 // -0*05", — 4 sc., 6 sc,; 
0*02"—0*6", —8 sc., 10 sc.; 0*02 // -0*8 // , -12 sc.; 0*02"-0T" —16 sc. 
The experiments were made consecutively, that is, on successive days, and, with 
the exception of Experiment IV., under precisely similar conditions. In Experi¬ 
ment IV. the temperature was inadvertently allowed to rise to 36°,* a fact which 
sufficiently accounts for the somewhat earlier beginning of the variation. 
The experimental results which have been given justify the statement (l) that 
the first phase of the variation is entirely negative ; (2) that it begins (at the tempe¬ 
rature of 32° C.) about one-twentieth of a second after the excitation; (3) that its 
electromotive force rises rapidly at first and subsequently very gradually; (4) that 
it culminates about the end of the first half-second after excitation and subsides 
gradually towards the end of the third half-second. 
In the paper which I communicated to the Royal Society with Mr. Page in 1876, 
I stated that the variation of the leaf did not begin until two-tenths of a second after 
excitation, and gave the grounds on which the statement was based. The experiments 
which led us to conclude that what we then called the “electrical delay” (the interval 
of apparent electrical inactivity) were correctly made. We repeated them under 
the most favourable circumstances in the Kew Laboratory in 1878,f with perfectly 
similar results, but nevertheless our interpretation of them was erroneous. 
* It is a probably also worth noting that the day on which this experiment was made was the hottest 
of last summer, Friday, July 16. 
| The observatioms made at Kew, in 1878, gave 0 - 12" as the shortest period of delay when the 
