258 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
An electric lantern should, as a rule, be fitted with a plain, colourless,
glass front, cylindrical in form, which should be at least 5/16 of an inch
in thickness, in order to obviate the probability of breakage. The
glass should be highly polished and free from air bubbles or other
visible defects. A dioptric lens may be fitted instead, if the owner so
desires, but where this course is followed the lens must be of an approved
type, and the lantern must comply strictly with para. 24.
19. Voltage.—The voltage of the electric supply on board should be
not less than 110 or greater than 220 volts The supply of electric
current for navigation lights should be kept at its full voltage throughout
the voyage, and opportunity should be taken to point this out to
masters of ships, as under-running of the voltage may result in serious
loss of candle power.
20. Candle Power.—Either 60 watt or, preferably, 100 watt lamps,
which will give candle powers of, approximately, 50 or 80 candles
respectively, should be used.
21. Form of Lamps.—Metal filament lamps should be used in all
cases, as carbon filament lamps of the candle power required involve
risk of serious over-heating.
Lamps should be of the normal type, having the usual cylindrical
("squirrel cage9') form of filament.
The diameter of the cage formed by the filament should be not less
than 1 inch (see para 26) and not greater than If inches.
22. Double Filament Lamps.—The use of double filament lamps, one
filament of which is intended for working purposes and one for emer-
gencies if the former fails, is not permitted, as it is impossible to obtain
an adjustment of the screening which will be correct in relation to
both filaments.
23. Gas-filled Lamps.—The Board of Trade are advised that these
lamps, as at present manufactured, 'sue not suitable for use in ships'
navigation lanterns. Their use, accordingly, should not be permitted
without prior reference to the Mercantile Marine Department.
24. Position of Electric Bulb.—The upright position (pip upwards)
should be adopted in all cases.
If the lantern used with an electric lamp is fitted with a plain glass
front, the exact height of the luminous centre of the source of light, in
relation to the centre of the glass, is immaterial.
If. hxjwever, an electric lantern is fitted with a dioptric lens, the
height of the socket is to be so adjusted that the luminous centre of the
source of light coincides with the centre of the lens.