CONTINUOUS-CURRENT DYNAMOS. 313 



now in use at the Carnaby Street station of the St. James' 

 and Pall Mall Electric Light Company, and of which mention 

 has already been made as having been exhibited at the 

 Naval Exhibition in London. These give an output of 

 1500 amperes at 120 volts or 180 kilowatts at a speed of 

 350 revolutions per minute, and experience has shown them 

 to be most satisfactory, both as regards heating and spark- 

 lessness. Their output works out to over 520 watts per 

 revolution per minute, and it is noteworthy that this is 

 attained by the use of a treble-wound armature, i.e., on each 

 armature core there are three separate and distinct wind- 

 ings, their commutator segments occurring in regular suc- 

 cession one after the other, so that every fourth segment 

 belongs to the same winding. Thus the current is collected 

 by two sets of wide brushes resting on the joint commutator, 

 and each set covering not less than three segments simul- 

 taneously. It would certainly appear that, by this device 

 of winding the armature so that there are more than the 

 two usual halves in parallel, a larger output may be ob- 

 tained without trouble from sparking or heating, and that 

 therefore the limit of output within which the two-pole 

 dynamo with its single horseshoe field may be used is 

 extended. Still there must come a point when recourse 

 should be had to a multipolar design, although it is still 

 an open question at what point this limit is reached. On 

 the Continent, as has been already said, multipolar types 

 have found more favour than at home and are more gener- 

 ally used for smaller outputs. In foreign central stations, 

 the dynamos are usually of larger size than with us and are 

 coupled to engines of the slow-speed double-acting class. 

 In a large number of cases they belong to one or other 

 of two types, manufactured respectively by the firms of 

 Siemens & Halske, and Schuckert & Co. The former has 

 internal radial magnets surrounded by a ring armature, 

 this latter being built up of segmental plates and supported 

 on bolts from a hub so as to overhang the magnet system. 

 The Berlin central stations contain the largest examples of 

 this type. At the Markgrafen Strasse station there are 



four such machines each of 400 horsepower at 80 revolu- 



22 



