TRANSMISSION OF POWER FOR SUBSTATIONS. 143 
ate size. When, however, the individual transformers can 
be of cousiderable output, say, fifty to one hundred kilo- 
watts, further increase in size is less important, for beyond 
such outputs the increase in efficiency and decrease in 
relative cost is much slower than the variation in smaller 
sizes. 'The increase of efficiency in passing, for instance, 
from a ten kilowatt transformer to one of one hundred 
kilowatts is about 2 to 214 per cent, while in passing from a 
one hundred kilowatt transformer to one of 500 to 1000 
k. w. the increase in efficiency is decidedly less than one 
per cent. 
With respect to the treatment of high voltage trans- 
mission wires after leaving the stations, much might be 
said, but since ordinary precautions for high voltage are 
obvious to the engineer it is sufficient to emphasize a few 
points dictated by experience. 
As to poles, as in many other matters, in the long run 
the best is the cheapest. Clean selected cedarisby far the 
best material available in this country. The poles for 
power transmission work should be somewhat longer and 
stronger than usual, for they have to carry substantial 
wires, often through open country where the wind has 
full sweep. The length should be great enough to keep 
the wires well out of the way and to permit carrying the 
lines easily over ordinary circuits when crossing is abso- 
lutely necessary. A good standard is a forty foot pole 
with a seven inch top, set fully six feet in well tamped 
earth. Now and then for runs across clear country thirty- 
five foot poles may well be used, but the tops should gener- 
ally be seven inches, and the setting about five feet six 
inches deep. The poles should run not less than about 
fifty to the mile. Such a line may appear needlessly 
heavy, but inasmuch as continuity of service depends on 
the integrity of the line, the precautions are well taken. 
For the same reason the cross arms should be extra strong 
and secured tothe poles with special care. 
The matter of insulators is of the utmost importance. 
Up to 4000 or 5000 volts large, strong, double petticoat,