20 ALTERNATING GENERATORS AND SYNCHRONOUS’ ' MOTORS. 
decrease the efficiency ; therefore it is better to use several 
parallel connected conductors. For this purpose special 
cables are very useful, as they can be pressed into the 
proper shape suitable for the slot. For high ténsion 
machines round wires are the most suitable. 
40. Insulation.—In alternating current designs where the 
| pressure ranges from the smallest value up to 20,000 volts 
or more, the question of insulation becomes a most im- — 
portant one. To secure good insulation between the arma- 
ture winding and the iron core, the coils are placed in the 
slots covered with a certain quantity and quality of in- 
sulating material according to the voltage of the machine, 
and the whole is again held in a fixed position by means of 
fibre or wood wedges, which prevent the coils from shifting. 
41. Quality of Insulation.—The material placed between 
the coils and iron core must have the following qualities: 
(a) Uniform and high insulating qualities, so that the 
thickness can be reduced to a minimum. 
(b) The variation in quality with the temperature to be a 
minimum, so that the temperature of the machine may be 
driven as high as possible. 
(c) It should not be affected by any mechanical treatment, 
but possess a high mechanical quality; for instance, it 
should be possible to bend it on a small curve’ without 
losing its good properties. 
(d) The material should not be ins epace but able to 
stand damp as well as dry air. a2 
42. Mica.—The insulating qualities'of mica have proved to be 
the best, but it has very little mechanical strength and is 
extremely brittle besides being expensive. 
For medium as well as for high tension machines, the usual’ 
material adopted is micanite, a flexible material which is 
made up of small sheets of mica pasted, pressed and 
baked together, with as little varnish as possible. The 
