16 ALTERNATING GENERATORS AND ‘SYNCHRONOUS MOTORS. 
0.02 inch in thickness and each sheet is insulated from 
its neighbor by paper or varnish, so as to minimize the 
eddy current losses. 
31. Windings.—tThe slots or holes on the inner periphery of 
the iron take the armature coils, which in case of open 
slots are wound on a wooden former to template, so as to 
insure interchangeability. 
In winding the coils each layer is covered with varnish; 
afterwards they are thoroughly dried to expel all moisture, 
in order to prevent any internal breakdown in the coil. 
This drying process can be advantageously performed by 
applying the short ecireuit eurrent for a few minutes, and 
repeating the performance several times. 
The coils are held in place by means of wooden or fibre 
wedges, 
32. The Core.—The armature core is generally built up of 
sheet iron, 0.01 to 0.02 inch in thickness, insulated with 
paper or varnish. 
The most reliable insulation material is paper of about 0.002 
inch in thickness, which can be cut in proper dise or 
segment form and stuck to the sheet iron, this work can 
be performed by means of a special machine. The pro- 
portion of the effective iron volume to the total volume 
of the armature core is about .85 to .90. 
oo. Method of Building.—In building the armature core, 
special care has to be taken in providing solid cheeks or 
end plates. In order to do this, stampings having a thick- 
ness of 0.1 inch are placed on the cheeks of the iron core, 
so as to give solidity to it, thus preventing the teeth of 
the thin sheet iron stampings from bending over under 
the pressure of the clamp rings or bolts. 
It should not be possible to drive the edge of a knife or 
finder between the stampings of a finished armature. 
The inner surface of the armature core need not be ma- 
