I42 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
horse power being ample to supply the air for cooling 
transformers of some hundreds of kilowatts capacity. 
Such air cooled transformers are capable of giving a 
large output for their weight and a very high efficiency. 
The average weight runs about twenty to twenty-five 
pounds per kilowatt of output, while the efficiency reaches 
and sometimes exceeds ninety-eight per cent. 
There is no difficulty in constructing these large sub- 
station transformers to give 10,000 volts or more from the 
high voltage coil and their construction is such that they 
are little subject to accident. ‘The air blast transformers 
separate the primary and secondary coils by air spaces and 
heavy mica insulation, while those in which oil is employed 
add its very high insulating properties to those already ob- 
tained from the construction of the transformers. Either 
type is thoroughly reliable for substation working. ‘These 
high voltage transformers should always be placed in a room 
by themselves, out of reach of all save the employes whose 
regular work it is to care for them, for 5000 to 10,000 volts 
means danger and should be treated with due respect. At 
such voltages no ordinary insulation is any guarantee of 
safety and bare wire which bears evidence of danger on its 
face is quite as desirable as any insulated wire. 
Perhaps the best plan for taking care of extreme volt- 
ages in generating or substations is to isolate them and keep 
them out of reach as far as possible, using switchboards with 
no exposed wiring on their faces. What wiring is necessary 
should be on porcelain insulators, not crowded, and per- 
fectly accessible when occasion demands, but not other- 
wise. Particular care should be taken to have the course 
of high tension wires obvious at a glance, avoiding all in- 
volved connections, so that it will be possible to trace at 
once every such wire from its origin at the high tension 
terminals of the transformers through the switchboard, if 
there be one, and safely out of the building to the line. 
Bear in mind that for the sake of simplicity, economy 
and efficiency, the transformer units should be few in 
number and of large size rather than many and of moder- 
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