Magnetic Circuit of Dynamo Machines. 505 



subjected in machinery materially diminishes the permeability 

 of iron, it is obvious that all joints in the magnetic circuit 

 ought, as much as possible, to be got rid of, consistently with 

 easy manufacture ; or, when they exist, they ought to be 

 made of as large area as possible, by letting one piece into 

 another to a considerable depth. 



In the design of a machine, the question of possible leakage 

 is of considerable importance. It will often be found that in 

 the effort to make I short and a 2 large, machines are designed 

 with enormous amounts of leakage not only from one pole to 

 the other, but from either pole to the middle of a neighbour- 

 ing field-magnet coil. 



One of the best ways of finding the nature of the probable 

 magnetic leakage in a dynamo, before it is constructed, is to 

 construct a small model of the same kind of iron, exciting the 

 field-magnets, and exploring by means of a ballistic galvano- 

 meter. Another simpler way, which gives a considerable 

 amount of information, and which we have employed, is to 

 make a model of wood, covering certain judiciously selected 

 parts, such as the poles and armature, and half the field- 

 magnets with metal, immerse the model in a barrel of rain- 

 water, and find the electric resistance between one part 

 and another when electric potential-differences are established 

 between them. On account of the ease with which the model 

 may be rearranged in configuration, this method of working 

 gives interesting results ; but these results, when applied in 

 the magnetic case, must be used judiciously and with the 

 knowledge that the permeability of iron is not a fixed quantity. 



It is obvious that the best section of a field-magnet limb is 

 the circular, but considerations of possible leakage to the 

 middle of the limb from the armature or a pole-piece, and 

 other considerations relating to the configuration of the 

 machine, often cause us to give to the section a rectangular 

 or oval section. 



The Characteristic of a Dynamo. 

 In a letter written by one of us from Japan, in January 

 1879*, before he had seen a dynamo, the necessity was shown 

 for establishing an algebraic relation between the E.M.F. 

 developed in the armature and the current exciting the field- 

 magnets. It was pointed out that E the E.M.F. in the 

 armature was proportional to the field and speed, and that 

 this led to nr 



E^ + ^+2^ (14) 



* Not published till 1885, and then in a somewhat mutilated form in 

 the ' Electrician ' of November 20. 



