91 
of Edinburgh^ Session 1878 - 79 . 
From the graphic representation of the results the following num- 
bers were taken : — 
Nominal Speed. 
Whole Resistance. 
Electromotive Force in 
terms of a Bunsen cell. 
800 
1-5 
38* 
3- 
38 
4-5 
36 
6 
31 
533 
1-5 
24 
3- 
23 
4-5 
17*5 
6' 
9- 
320 
1-5 
13 
3* 
5 
. . . 
4-5 
2*5 
6 
2* 
Next, instead of the second or third coils above mentioned, a 
Duboscq’s lamp was included in the circuit, the other arrangements 
being as before, and the speed being 800 nominal. The deflection 
corresponded to an electromotive force of about 39 Bunsen cells, 
and a resistance of 2*66. As the lamp itself was sometimes found 
to have a resistance of as much as 0’6, and as the carbons have 
a resistance of from 0T15 to 0045, per 4 inches, it appears that, 
approximately, the resistance of the electric arc, under these condi- 
tions, is at least 0*8. 
Subsequent experiments, in which the lamp was not used, gave re- 
sistances varying from 0 - 75 to 1*2, according to the length of the arc 
— and when a little sodium was introduced, it fell to 0*25. These 
estimates, of course, include the effect due to heating and pointing 
the carbons. 
The want of accurate speed determinations, of course, deprives 
these results of scientific value, but they are very useful as an 
expression of the electromotive force practically to be obtained from 
the Gramme machine under different circumstances of its ordinary 
working, — showing, as they do, what adjustments to make for the 
purposes of a particular experiment. 
* This particular number must be over-estimated, for about 5 h.p. is re- 
quired to maintain an electromotive force of 38 Bunsens in a resistance 1 '5. 
