146 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
in which a voltage of 20,000 is designed to be used. The
insulators were tested at 40,000 volts and very few had
to be rejected for failure. They depend for their insu-
lating power on the quality of the porcelain and on well
designed double petticoats. Such insulators are admir-
ably adapted for work up to at least 15,000 volts, pro-
vided the climate is reasonably
good. In very moist climates
where the insulators are exposed
to frequent searching mists and
nearly constant dampness still
further precautions are desirable,
\
|
Z l’j and probably the best results can
. e
//// il be secured by using oil insulators
| of which a very good specimen
""| is shown in section in Fig. 84.
i
I The pin, P, of iron, is cemented
'W into the body of the insulator, I,
”“;{ which is made thick and solid.
The thick bell of the insulator is
turned inwards and upwards at
its lower edge so as to form a cir-
cular cup, C. 'This cup is filled
with highly insulating oil, which
is exceedingly efficient in stop-
ping leakage along the surface of the insulator to the iron
pin. Insulators similar to this were used in the famous
Lauffen-Frankfort experiment and proved to be excellent.
In dry and dusty weather, however, the oil accumulates
dirt and is likely to be reduced to a species of mud, quite
destroying its insulating value.
With respect to the general arrangement of a trans-
mission line too much care can hardly be taken in keeping
the circuit away from danger of accidental contact to per-
sons and things. Bare wire is preferable to insulated since
it does mot encourage a feeling of false security, and it
should be distinctly understood that the wires are danger-
ous and must be let alone. Particular pains should be
FIG. 84.