118 



ARMATURE CONSTRUCTION 



cement and moisture, and assists in making the structure compact 

 and solid. It must, however, not be carried to such an extent as 

 to carbonise the cementing varnish ; for in this case any oil in 

 proximity to the mica insulation would be absorbed, and ultimately 

 lead to the disintegration of the mica plates. Some firms supply 

 built-up mica plates, for which they claim that the adhesive 

 matter in the finished plate does not exceed 1*25 per cent, of the 



FIG. 135. Typical modern large Commutator Construction. 



weight. The product of different manufacturers varies greatly in 

 this respect, and comparative tests of various qualities as regards 

 softness, percentage of adhesive matter, and heating, when 

 subjected to a high potential test from side to side, are very 

 desirable before determining upon the quality to be employed. 



As before stated, with the rapid increase in the size of the 

 commutators, the use of hydraulic pressure has been more 

 or less given up ; it would, nevertheless, appear to be of advan- 

 tage in the interest of obtaining the greatest possible solidity. 

 Hydraulic pressure should also be employed in pressing up the 

 armature laminations, and machinery suitable for both these 



