90 Proceedings of the Royal Society 
5. Laboratory Notes. By Professor Tait. 
(1.) Measurements of the Electromotive Force of the Gramme 
Machine at Different Speeds. 
The following measurements were made by means of a Gramme 
machine, recently procured for the University. I desired to make 
use of it, not only for electric light, but for electrolysis, the excitiug 
of electromagnets, and various other purposes for which we have 
hitherto used from 4 to 40 or so Bunsen cells. I therefore 
arranged the driving-gear so that with the same motor (a 3J h.p. 
gas-engine) it was easy to use either of three speeds. These are, 
approximately, 800, 533, and 320 turns per minute. The electro- 
motive force varies, of course, not only with the speed but with 
the resistance of the whole circuit — falling off at first rapidly and 
then more slowly for any one speed, as the resistance is increased. 
As I had no means of measuring the speed directly , I was some- 
what puzzled at first to find the electromotive force at any one 
speed rise to a maximum, and then rapidly fall off as the whole 
resistance was gradually diminished. But I soon found that this 
was due in great part to the slipping of the driving-belts (though 
they were very tight), whenever the intensity of the current exceeded 
a certain amount. A liberal use of rosin almost removed this anomaly, 
though there is reason to believe there is still considerable slipping. 
The following table gives the average of a number of experiments 
which accord fairly with one another. The resistance of the con- 
ductor of the Gramme machine is about 1T6 B. A. units. For the 
added resistance I used coils of stout covered copper wire which 
were in the laboratory, having resistances 0*054, 1*844, and 3*63, 
respectively. The first was always in circuit, and a portion of it, of 
resistance 0*0015, was introduced into the circuit of a galvanometer 
having a resistance 23. The deflections of the galvanometer were 
observed with the first coil alone in the circuit of the Gramme, then 
with the addition of the second or third,, and finally with all the coils. 
The explosions in the gas-engine occur only at every second stroke 
of the piston. This and other causes render the driving power not 
absolutely steady, but the average deflection of the galvanometer 
was very easily observed. 
