340 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



In either case, the current is broken at the instant represented by the 

 current-curve crossing the axis of X. The intervals A' B', A"B", etc. 

 in Figure 1, are the intervals durinij which the current is heating the 

 arc ; while the intervening intervals B' A", B"A"', etc., are the intervals 

 during which the visual or photographic examination of the arc is made. 



Figure 1. 



The interrupting disks, placed in circuit one on each side of the arc, 

 are made of rather heavy brass rings, 30 cm. in diameter, and insulated 

 from the shaft on which they rotate. Into the faces of these brass rings 

 are set two slate sectors, each 45° in length, as represented in Figure 2. 



A pair of large brush holders is centred on the same shaft wrih the 

 interrupters. Each holder carries two brushes, which bear on the inter- 

 rupter at points 90° apart. Since the dynamo is an eight-pole machine, 

 it is evident that the current will be " on " half the time and " off " half 

 the time, both sides of the circuit being opened and closed alternately at 

 the end of each 45° of rotation. 



FiGUBE 2. 



The brush holders are, of course, adjustable so that they may be set at 

 a position of minimum sparking. But one defect of this method of inter- 

 ruption is that the brushes must have a finite thickness, and hence can 

 never cross the boundary between brass and slate without some sparking. 

 Not only so, but as the phase of the current changes with load to a slight 

 extent, the angular adjustment of the brushes can never remain perfect. 

 To reduce these changes of phase to a minimum the terminals of the 

 dynamo were kept on a closed circuit all the while, the poles of the 



