DIRECT-CURRENT DYNAMOS AND MOTORS. 5 
the coils which differ considerably in potential are widely 
separated from one another. For this reason the ring 
armature is preferable for very high voltage machines, 
such, for example, as_ series are lighting generators. 
Formerly the ring armature had the advantage over the 
drum that single coils could be taken off and replaced 
without disturbing the rest of the winding; but the gen- 
eral use of formed coils for drum armatures, which is 
now the common practice, enables the same thing to be 
done in their case with almost equal convenience. 
The great advantage of the drum form is due to the fact that 
it can be directly and securely mounted upon the shaft. 
In some cases armature cores are carried upon spiders, so 
that they may be wound in either the ring or drum 
fashion as desired. The latest practice seems to tend 
toward the adoption of the drum in preference to the ring 
type for small and medium size machines. This matter 
will be understood more clearly when the details of form 
and construction have been more fully considered. 
%. Armature Cores.—If the core of the armature were 
made of one solid piece of iron, currents would be set up 
in it, since the outer portions of the core cut magnetic 
' Sasi lines, exactly as the armature conductors themselves. 
. The useless currents thus generated, called Foucault or 
eddy currents, consequently flow in the same direction as 
the armature currents, that is to say, ordinarily in a di- 
rection parallel to the shaft of the armature. In order to 
prevent these eddy currents, which consume power and 
also cause heat, from flowing, it is, therefore, necessary 
to intercept their path by subdividing, or laminating, 
the core perpendicularly to the shaft. 
The usual plan is to build up the core of discs or rings of 
sheet iron, lightly insulated from one another, so that the 
magnetic lines can pass freely through each disc, but the 
eddy currents which tend to flow perpendicularly across 
the discs are stopped by the insulation between the lat- 
ter. Since the eddy currents tend to flow in the same 
