10 



Extracts from Report of the Committee on 

 Dynamo -Electric Machines of the Franklin 

 Institute of Philadelphia. 



The Brush machine has, for its magnetic field, two horseshoe 

 electro- magnets, with their like poles facing each other, at a suit- 

 able distance apart, the circular armature rotating between 

 them. 



In this machine the currents are generated in coils of copper 

 wire, wound upon an iron ring, constituting the armature. This 

 ring is not entirely covered by the coils, as in the Gramme arm- 

 ature, but the alternate uncovered spaces between the coils are 

 almost completely filled by iron extensions from the ring, thus 

 exposing large surfaces of the armature ring for the dissipation 

 of heat, due to its constantly changing magnetism, as in the 

 Pacinotti machine. 



The ring revolves between the poles of two large field mag- 

 nets, the two positive poles of which are at the same extremity 

 of the diameter of the armature, and the two negative poles at 

 the opposite extremity, each pair constituting practically ex- 

 tended poles of opposite character. 



The coils on the armature ring are eight in number, opposite 

 ones being connected end to end, and the terminals carried out 

 to the commutator. In order to place the commutator in a con- 

 venient position, the terminal wires are carried through the cen- 

 ter of the shaft to a point outside the bearings. 



The commutators are so arranged, that, at any instant, three 

 pairs of coils are interposed in the circuit of the machine, work- 

 ing, as it were, in multiple arc, the remaining pair being cut out 

 at the neutral point; while in the Gramme machine, the numer- 

 ous armature coils being connected end to end throughout, and 

 connections being made to the metal strips composing the com- 

 mutator, two sets of coils in multiple arc are at one time inter- 

 posed in the circuit, each set constituting one-half of the coils on 

 the armature. 



The commutator consists of segments of brass, secured to a 

 ring of non-conducting material, carried on the shaft. These 



