ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 27 
shaft which is run over a pit and is completed in this posi- 
tion without taking it from the shaft. This method of 
machining and building results in the perfectly true run- 
ning of all parts of the whez2l. 
54. Field Coils.—tThe field coils of smaller sizes are generally 
hg wound with double cotton covered copper wire and those 
of larger sizes are preferably wound with copper strip on 
edge, the turns being insulated from each other with paper 
or micanite. 
The insulation between the coils and the pole core is in most 
eases press-board or paper, which has very good mechanical 
qualities, for such are required on the revolving part of a 
generator. The coils being former wound and interchange- 
able are held in position by means of the extension of the 
pole pieces, the coil can be replaced by taking adrift one 
magnet limb, with its coil, from the wheel. 
Generally a double covering of the wires is sufficient and only 
in the case of high tension excitation or high peripheral 
speeds are wires of triple cotton covering or a double cover 
of spinning and one cover of braiding used. 
55. Exciting Current.—The current for exciting the field 
magnets is conducted through brushes to two slip rings, 
which require to be of ample dimensions in order to carry 
the rated current without undue heating. It has been 
found an average good value to employ a current density 
of 100 amperes per sq. in. between the gunmetal slip rings 
and carbon brushes, and 400 amperes per sq. in. if the 
brushes are of copper. 
.56. Material and Labor Costs.—As already pointed out, 
the designer has to study the choiee of the material, the 
eonstructional details, the workshop methods, the material 
and labor costs, in order that he may be able to’ judge if, 
with his best electrical and magnetic design, he can realty 
obtain the cheapest machine. : 
