Electromagnetic Induction Machines. 



443 



thods of magnetization. The 

 annexed figure, representing an 

 end view of the cylinders with 

 the armatures in section, will 

 make this arrangement of the 

 cylinders pretty clear to those 

 who are familiar with the con- 

 struction of these machines. 



The residual or permanent 

 magnetism of the large electro- 

 magnet with its cylinder is very 



considerable, being many times greater than that of the four 

 small permanent magnets with which it was originally excited*. 

 The coils of the small armature are placed in connexion with 

 those of the great electromagnet ; and when the armature is ro- 

 tated, the magnet-cylinders act and react on each other until the 

 electromagnet is excited to the highest degree of intensity. By 

 this arrangement of the armatures and cylinders, the minor cur- 

 rent for exciting the electromagnet is kept distinct from the 

 major current from the larger armature, which may be coiled 

 for currents of high or low tension according to the purpose for 

 which they are required. 



It is essential for the attainment of a high degree of magne- 

 tism in an electromagnet excited by magneto-electricity, that the 

 continuity of the armature and electromagnetic circuits should 

 be preserved during the change of contacts from one segmental 

 part of the commutator to the other. For this purpose the seg- 

 ments are made to overlap each other for a short distance, so 

 that the metallic rubbers or brushes for taking off the current 

 bear on adjoining segments simultaneously at the point of no 

 current, and, in so doing, form two closed metallic circuits for a 

 brief interval, which may be represented by the numeral 8, the 

 upper part of the figure representing the armature circuit, and 

 the lower part that on the electromagnet ; but when the arma- 

 ture is at that part of its revolution when the current begins to 

 rise in intensity, the coils of the armature and electromagnet 

 form one continuous circuit, which may be represented by the 

 cipher 0. The importance of keeping the circuits closed in the 

 manner described was not sufficiently observed in my earlier 

 experiments, and necessitated the employment of much more 

 powerful currents for exciting the electromagnets than were 

 afterwards found to be necessary. 



* The small scale upon which my experiments have been repeated by 

 physicists has in some instances given rise to the idea that the residual 

 magnetism of an electromagnet is a lower degree of permanent magnetism 

 than that which originally formed the basis of my augmentations. 



