ALTERNATE CURRENT DYNAMO-ELECTRIC MACHINES. 
OQ Q 
brush-holder, so that one presses on each ring. The circuit between the two brushes 
is completed once in each revolution, and the position of the contact can be read off 
by a pointer attached to the holder on a circle 13| inches diameter, divided into 
360 degrees. The two points between which it is desired to measure the potential 
difference are connected through the contact-maker to a condenser and the quadrant 
electrometer, as shown in fig. 2, in which A and JR are the points, C the revolving 
A 
3 
B 
0C 
3 =© 
Fz8Z 
Fiz3. 
contact-maker, D the reversing switch of the electrometer, E the condenser, and F 
the quadrant electrometer. By plotting as ordinates the volts measured at any 
epoch, and as abscissae the position of the contact-maker as representing time, the 
curve of potential is obtained. The electrometer is standardised by means of a 
Clark cell, so that the deflections on the scale can be reduced to volts : when the 
potential difference between A and B was too great for the electrometer, it was 
reduced in any desired ratio by two considerable non-inductive resistances introduced 
between A and B, as shown in fig. 3. 
Eig. 4 . 
The characteristic curve of the alternator is given in fig. 4, and shows the relation 
between the total induction I, between one pole piece and the opposing one, in terms 
of the line integral of magnetising force due to the two windings in series on the two 
respective magnet cores; the scale of amperes in the magnet winding is also given 
horizontally. 
MDCCCXCVI.—A. 2 H 
