3 i2 SCIENCE PROGRESS. 



engine at the slow speed of 300 revolutions per minute. 

 The Colossus machine was constructed for reducing 

 aluminium ore in an electric furnace, and the require- 

 ment of very large currents for electric smelting by the 

 Cowles or Herault processes has called forth some of the 

 largest continuous-current dynamos in existence. In 1888 

 we find the electro-metallurgical works at Lauffen-Neuhausen 

 using a 300 horsepower turbine to drive at the slow 

 speed of 180 revolutions per minute two six-pole 

 Oerlikon dynamos, each supplying 6000 amperes at 20 

 volts, or 1 20 kilowatts : each of these dynamos in reality 

 consisted of two armatures wound on the same core, a 

 commutator with six sets of brushes being placed on either 

 side of the machine. In the same year, the Cowles 

 Aluminium Company had at work a two-pole dynamo 

 giving the very large output of 60 volts and 5000 amperes 

 at 380 revolutions per minute. More recently the Oerlikon 

 Company have constructed large multipolar machines also 

 for the smelting of aluminium ores at Neuhausen, in which 

 the armature is mounted on the extended shaft of a turbine 

 and revolves in a horizontal plane within a ring of poles 

 pointing radially inwards. The latest (1893) of these give 

 an output of 7500 amperes at 55 volts or 550 horse- 

 power when running at 150 revolutions per minute: they 

 have twenty-four poles and a huge commutator from which 

 the current is collected by 1 20 brushes in twenty-four sets ;. 

 the total weight of turbine and dynamo amounts to as 

 much as twelve tons, and the turbine is arranged to exert 

 an upward thrust so as to take a part of this weight off the 

 footstep of the vertical shaft. 



The comparative performance of machines in relation 

 to their size is best estimated by the number of watts per 

 revolution per minute which they respectively give, the 

 speed at which any output is obtained being in fact an all- 

 important item in their comparison. Judged by this test, 

 the output of the Cowles dynamo alluded to above, viz., 440 

 watts per revolution per minute, must be regarded as very 

 large for a two-pole dynamo. Probably the largest con- 

 tinuous-current two-pole dynamos in existence are those 



