86 DYNAMO-ELECTRIC MACHINES. 



will be, consequently, as many currents in alternate directions in one 

 revolution of the armature, as there are poles. These machines were 

 first brought out, it is said, by both Gramme and Siemens. The 

 Jablochkoff candle, which you have heard spoken of so often, in 

 connection with the lighting of the streets at Paris, and the first 

 attempts at lighting the streets of London with electricity, had the 

 currents supplied to them by machines of this kind. Alternate 

 current machines have been very little used on this continent, and 

 they are giving way to those with continuous currents in Europe. 

 The light they furnish per H. P. absorbed, is much less than that 

 given by continuous current machines. They are somewhat simpler 

 in construction, as the commutator, composed of many pieces, in the 

 continuous current class, is replaced in the alternate current, by two 

 plain discs, upon each one of which the brushes rub. For this among 

 other reasons, they are said to be more durable. With the exception 

 of the winding of the armature and the arrangements of the magnets, 

 the Siemens is almost identical with the Gramme. The Weston is 

 identical with the Siemens outwardly, the only difference being in 

 the material of the magnets, the former being of cast iron, and the 

 latter of wrought iron, and the arrangements of the bearings for the 

 armature. There is no doubt but that all machines in use are exact 

 copies of either the Gramme or Siemens in principle, modified more 

 or less in detail only. The system of winding the armature and of 

 attaching to commutator, the construction of commutator, and the 

 method of collecting the electricity as generated by brushes, was 

 wrought out entirely by Gramme. Every type of machine has copied 

 this. In the early history of the Siemens they attempted to use 

 rollers for this purpose, but failed, owing to the impossibility of 

 making good contacts with a solid piece of metal, as is done by 

 brushes made of thin sheets of copper or a number of fine wires, and 

 the rollers had to be abandoned. All the successful machines at 

 present in use are more or less copies of the two classes mentioned. 

 The closer the copies to one or the other, the more satisfactory have 

 been the results. All have not been equally successful. Some of 

 the features of the Brush are objectionable, although it has been one 

 of the most successful before the public. The Weston machine is 

 very highly spoken of, and is a very close imitiation of Siemens. 

 The Edison has the Siemens method of winding the armature, but a 



