134 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
insulation of the line and the generator or transformers, 
which is a very different matter. 
Hence while for railway motor service most of the 
work has been at about 500 volts, and 1000 volts appears 
to be the extreme limit in the present state of the art, al- 
ternating power transmission lines are very generally 
worked at from 4000 or 5000 volts up to 10,000 or even 
more. 'The transmission of power to the Oerlikon works 
near Zirich, Switzerland, has been steadily operated for 
several years at about 14,000 volts on the line, while those 
at San Bernardino, Sacramento, and Fresno, Cal., are 
operated at from 10,000 to 11,000 volts. ‘This means that 
the line copper required is less than one per cent of that 
which would be needed for direct feeding of the motors at 
the same percentage of loss. For railway work in distrib- 
uting power to substations nothing less than 5000 volts is 
likely to be used and 10,000 will be frequent. At dis- 
tances at which substation working becomes economical 
less than 5000 volts will hardly pay. We have already 
seen in the preceding chapter that on anything less than a 
fifteen or twenty mile road, transmission to sub-stations is 
not likely to compete advantageously with the ordinary 
device of separate stations. At such distances 10,000 volts 
is to be recommended much oftener than s5o000. 
The problem of getting such voltages is not altogether 
simple. The most nsual method is to generate the power 
at a rather moderate voltage, say, 500 or 1000, and then to 
obtain the high line pressure from raising transformers. 
For voltages of 10,000 and upwards this is by far the best 
plan, and so indeed it is generally for 5000 volts, but for 
pressures up to the last mentioned figure and even above it, 
there is a strong tendency to construct special high volt- 
age dynamos feeding directly into the line. ‘This avoids 
the cost of the raising transformers and the loss of energy 
incurred in them. On the other hand such high voltage 
dynamos are rather difficult and expensive to construct 
and somewhat more liable to deterioration than those of 
lower voltage. While it is possible to wind alternators in