66 
revolving for a few moments, the discs resumed their nor- 
mal positions with respect to each other (as indicated by 
the action of the clutch) ; thereby exhibiting, not only the 
synchronous rotation of the armatures, but also, that the 
machines contained a principle of self-adjustment to the 
position in which the maximum effect of the combined cur- 
rent was obtained. It was therefore evident that this 
property of the current, to maintain the synchronism of the 
armatures, rendered it unnecessary to employ mechanical 
gearing of any kind for that purpose. 
The author then proceeds to enter into a detailed explana- 
tion of the causes which produce the synchronism of the 
armatures, a summary of which is as follows : — When the 
armatures happen to be in that position during their revo- 
lution in which they are producing the maximum and 
minimum amount of current respectively (as must often be 
the case when there is no synchronism), that current which 
is at the maximum rushes through the coil which is pro- 
ducing the minimum amount of current, — the effect of this 
passage of the current from one coil to the other being to 
accelerate or retard the rotation of the armature (according 
to the direction of the current) until synchronism, is 
established. On the other hand, the absence of synchronism 
observed when the direct current was taken from the 
machines, by means of commutators, is caused by the direc- 
tion of the currents being coincident with that which they 
would receive by induction from the electro-magnets, and, 
consequently, opposite to that which tends to impart an 
accelerating or retarding impulse to the armatures. 
The synchronous rotation of the armatures having been 
secured in the case of the combined alternating currents, it 
yet remained to obtain the combined direct currents from 
the two machines. A pair of rings and a commutator were 
therefore fitted upon one of the armature spindles, which 
was made sufficiently long for the purpose ; and metallic 
