1885.] NEW YORK ACADEMY OF SCIENCES. 95 



force, enables us to concentrate this field, and to place it, so to 

 speak, where we desire to use it. 



Pare, soft, wrought iron may be said to be 20,000 times more 

 permeable than air. 



Yon will see from Fig. 4 what I mean. The wrought-iron 

 cores C afford the easiest path for the lines of force, and they 

 therefore follow them until they reach the armature space A, be- 

 tween N and S, where they take their airy path across, because 

 the lines of force must always close. 



"We see that we have thus managed to concentrate the lines of 

 the field of a large number of coils in a small space A. In this 

 space the wires of the armature are revolved so as to generate a 

 current which is either alternating, or approximately continuous. 



The details of armatures and winding of them, as well as of the 

 commutators, will be found described at considerable length in 

 the works on Dynamo Electric Machinery, written by Dr. Schel- 

 len, or Prof. S. P. Thompson. 



I do not think that there is anything written better calculated 

 to give the novice a clear idea of the principles involved in a con- 

 tinuous current machine, than Pacinotti's own description of his 

 machine, which can be found in the translation of Schellen, by 

 Dr. Keith, on page 209. 



I have shown you how perfectly the dynamo converts mechani- 

 cal into electrical power, and I will now have to confess to you 

 how exceedingly imperfect and irregular are the present methods 

 of transformation of heat into mechanical power. Engines and 

 boilers are both built in entire disregard of the actual principles 

 of thermo-dynamics, and there has been so little accurate knowl- 

 edge of the practical laws which should govern their construction, 

 that makers, in many instances, shelter their wretched designs 

 and workmanship behind the impossibility of predetermining the 

 proportions which will realize the greatest economy of heat. 



