6 _ DIRECT-CURRENT DYNAMOS AND MOTORS. 
direction as the useful currents in the conductors, the 
core should always be. laminated parallel to the lines of 
force and to the direction of the motion. 
8. Wire Cores.—Iron wire has been employed for arma- 
ture cores, and possesses the advantage of very com- 
plete subdivision and of being easily wound in a ring or 
drum of any desired size or form. A wire core has, 
however, the disadvantages of promoting magnetic dis- 
continuity, of constituting a poor mechanical construc- 
tion, and of entailing loss of space between the wires, 
especially if they are of circular cross-section. 
9. Core Dises.—Discs or rings are almost always used for 
armature cores, and are punched or cut out of the softest 
sheet iron or mild steel, a special quality being manufac- 
tured for this purpose. The thickness is usually from 
.01 to .03 inch, astandard size being .022 inch. The same 
thickness is often employed for all sizes of machines, in- 
cluding the largest as well as the smallest, being adopted 
because it is convenient to handle and to punch, and is 
sufficiently thin to reduce the eddy currents to a small 
value. 
For armatures less than about 2 feet in diameter, each core 
disc is a complete ring in one piece, but armatures of 
larger diameter are built up of a number of sectors in 
order to facilitate their punching and handling. 
10. Effective Cross-Section of Armature Core.—The 
: laminz were formerly insulated from each other by 
tissue paper, paint, varnish, or rust, but the usual 
practice at present is to simply rely upon the oxzde coat- 
ang on the surface of the discs. 
An armature core composed of such sheets, and forced to- 
gether by heavy hydraulic or screw pressure, is found to 
contain from 85 to 95 per cent. of its volume of iron, the 
rest being made up of insulation, scale, and small air 
spaces due to slight inequalities in the sheets; hence the 
