252 ARMATURE CONSTRUCTION 



weakness with thinner insulations, and this would ultimately 

 entail defective insulation. With this reservation, the above 

 three examples will serve as a guide by which other types and 

 sizes may be constructed. Great care must be taken to ensure 

 the high quality of all the materials employed. A great deal 

 depends upon having the varnishes adjusted to the most suitable 

 consistency. 



In the case of alternating-current machinery, still greater care 

 should in general be given to the insulation, owing to the higher 

 voltages generally employed, and the lower factors of safety 

 which it is practicable to adopt. When, however, the potential 

 does not exceed 500 volts, the insulation may be very much the 

 same as for continuous-current machines ; in fact, the present 

 custom is to permit a much lower insulation standard than for 

 continuous-current machines for the same rated voltage. This is 

 a wrong tendency, and has no possible justification. In fact, for a 

 given rated voltage, the maximum voltage occurring is higher in 

 the alternating-current machinery to the extent that the maximum 

 exceeds the R.M.S. voltage. 



For voltages above the range of those employed for continuous- 

 current machines, presspahn may in some cases preferably be 

 replaced by horn fibre, owing to its higher mechanical and dis- 

 ruptive strength. Indeed, if manufacturing companies were not 

 so hopelessly involved by tradition and established customs, the 

 better material would more often be used, and with advantage, 

 since any difference in the cost of the insulating material would 

 be of negligible influence on the total cost of the machine. This, 

 however, should not be taken as advocating so wastef ully expensive 

 a practice as the use of micanite linings on continuous-current 

 motors of from 250 to 600 volts. 



On alternating-current machinery of from 1000 volts upward, 

 a moulded presspahn-mica insulation is very suitable. In such cases 

 some firms employ micanite tubing. This is, however, not only 

 more expensive, but, what is most important, it is more liable to be 

 injured by moisture and dampness, which ultimately permeates the 

 sticking varnish and causes the tube to lose its compactness and 

 become mushy, ultimately disintegrating after long service in damp 

 situations. 



Presspahn-mica may be made up as follows: On a sheet of 

 oiled presspahn 0'2 mm. thick, one layer of flakes of rnica is pasted 

 slightly overlapping, over three-fourths of its surface, a rim equal 

 to one-fourth of the width of the sheet remaining uncovered. This 



