THE INSULATING OF ARMATURE CONDUCTORS 253 



sheet is rolled up over a mandril and baked in a form. The mandril 

 is of the dimensions desired for the inside of the tube. The tube 

 may be sawn open at the top with a circular saw. The slitted tube 

 is inserted in the armature slot, and the wires are fed in through 

 the slit. Before pasting the flakes of mica on the presspahn, the 

 latter should first be dried for three hours at 70 C. in a vacuum 

 oven, and then placed for twenty-four hours in a tank of pure, 

 double-boiled linseed oil, the oil being maintained at a temperature 

 somewhat below its boiling-point, by means of steam-pipes lining 

 the tank. The presspahn should then be removed and dried in 

 an oven or in the air the former being the quicker process, 

 but otherwise possessing no advantage, in such a case, over drying 

 in air. 



A presspahn-mica tube 2*5 mm. thick, consisting of seven 

 layers of presspahn and six layers of mica, should, if carefully pre- 

 pared, have a disruptive strength of 30,000 RM.S. volts. It has 

 been found that in the construction of micanite tubes the sticking 

 varnishes are the source of much trouble, and they should therefore 

 be used sparingly. The adhesive qualities of the linseed oil with 

 which the presspahn is impregnated, are sufficient, when the tube 

 is warmed up, to be depended upon for the purpose of sticking 

 together adjacent layers. If the ends of the tube are dipped in 

 hot paraffin, it will prevent moisture creeping in between the 

 leaves. Such a presspahn-mica tube constitutes a moisture and 

 acid-proof, non-deteriorating insulation, and has the advantage 

 over micanite tubes that the presspahn, being in one continuous 

 piece, holds the other components firmly together. In micanite 

 tubes, moisture penetrating into the sticker leads to disintegration. 

 So-called micarta and pertinax tubes have recently been placed 

 on the market by representative firms dealing in insulating 

 materials. The pertinax tubes contain no mica. 



-"-- " 



OF ELECTRICAL LABORATORY, 



FACULTY OF ALLIED SCIENCE. 



J 



FACULTY AL . 



u*u ............................ .......... ___________ ___________ *, 



