CHAP, x.] ELECTRICITY AND MAGNETISM. 375 



and the armature to rotate left- ban dedly, as seen in 

 Fig. 151, then, by the rule given in Art. 395, in all the 

 separate wires of the coils, moving from right to left over 

 the top, there will be currents induced in a direction 

 from the front toward the back. In all the separate 

 wires of the coils moving from left to right below the 

 axis, the induced currents will' be in a direction from 

 the back toward the front. Hence, if the coils are 

 appropriately joined to the commutator all the currents 

 thus generated in one half of the coils will be flowing 

 into the external circuit at one of the commutator 

 brushes ; and all the reverse currents of the other half 

 of the coils will be flowing out of the other brush. The 

 terminal screws connected by wires to the commutator 

 brushes correspond to the + and poles of a galvanic 

 battery, the coils of the field -magnets being included in 

 the external circuit. 



41O. Gramme's Machine. The dynamo -electric' 

 generator invented by Gramme differed from all previous 

 forms in the employment of a ring armature (previously 

 applied by Paccinotti in the construction of an electro- 

 magnetic motor) made of soft iron and overwound with 

 coils of insulated copper wires. The principle of this 

 generator is shown in diagram in Fig. 152. The ring 

 itself, made of a bundle of annealed iron wires, is wound 

 in separate sections, the ends of each coil being joined 

 to strips of copper which are insulated from each other, 

 and fixed symmetrically as a commutator around the 

 axis, like a split tube. Their actual arrangement is 

 shown again in Fig. 153. The coils of the separate 

 sections of the ring are connected together in series, 



horizontal in direction, changes the resultant magnetic force to an oblique 

 direction across the field. It is for this reason that the commutator " brushes " 

 have to be displaced with a certain angular "lead." A similar displace- 

 ment of the brushes occurs in the Gramme and all other dynamo-electric 

 generators, the degree of displacement to get maximum strength of current 

 varying with the resistances in the external circuit and with the work done 

 by the current. 



