ALTERNATE CURRENT DYNAMO-ELECTRIC MACHINES. 
241 
the leading machine being generator, and were connected in series with a non- 
inductive resistance r, and a Kelvin ampere balance C, as shown in fig. 13. The 
potential difference of the generator was measured at different epochs by means 
of the Kelvin quadrant electrometer Q, and the contact maker K. the potential 
applied to the electrometer being reduced by the non-inductive resistances r x , r 2 . 
For corresponding epochs a curve of potential was taken across r, this gives the 
current passing between the machines and also the difference of potential difference 
between motor and generator. 
The power difference, or loss in the combination, was supplied by a shunt-motor 
through shafting and belts, and was determined by observing the watts supplied to 
the motor when driving the shafting, the alternator belt being removed, and then 
observing the wmtts taken to drive the alternator when loaded—the speed being 
the same in each case. The difference gives the power absorbed by the combination. 
It was found that the watts required to drive the shafting alone were 1681 ; 
the watts required to drive the shafting and alternators when excited, but not 
loaded, were 2479, the difference being in part due to currents induced in the metal 
frames of the armature. 
Tables II. and III. give for about half and full load (with regard to current only) 
the data for getting at the watts given out by generator and received by motor, 
and have been obtained from the potential and current curves. The phase difference 
between the armatures was y^th and y^th period respectively. 
Table IY. shows how the efficiencies of generator, motor, and combination have 
been obtained ; and also gives the allocation of losses in the system. 
The frequency was about 70 periods per second, and, since the machines are built 
for 100 periods per second, the figures must be taken only as illustrative of the 
method of test. 
The alternators being connected, as shown in fig. 13, we were able to vary the 
exciting currents of the two machines by means of the adjustable resistances r 3 , r 4 . 
The armatures were coupled -y 0 -th of a period apart in phase, and the following 
experiments w T ere made. 
1. The alternators were equally excited with a current of 17'5 amperes and run at 
a speed of 716 revolutions per minute, corresponding with a frequency of 71’6 periods 
per second, and the following curves obtained (see fig. 14). 
E g , E m are the E.M.F.’s of generator and motor wdien running on open circuit. 
pd g , pd m are the potential difference of generator and motor respectively when a 
current of 42'2 amperes (\/mean 2 ) was passing through the armatures. 
e G , e M are the E.M.F.’s of the respective machines when loaded, that is to say, they 
are the curves PD G , PD M corrected for current into armature resistance. 
E is the E.M.F. of the combination when not loaded, that is, it is the difference of 
the curves E G , E M . 
x is the curve of current passing between the machines, and is proportional to the 
MDCCCXCVI.-A. 2 I 
