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48
lantern purposes generally has its carbons out of the
vertical and the negative carbon slightly in front of the
other, since by such
as it were, tipped
MODERN MAGIC LANTERNS.
Fig. 37. DAVENPORT'S ARC LAMP.
an
on
arrangement the crater becomes,
one side, so as to present its
interior towards the
condenser. This will be
best seen from Fig. 38,
in which A is the positive,
B the negative
carbon, c the crater, and
D the condenser. When
the current supplied is
not continuous but alternating,
it will not be
found so convenient for
lantern purposes the
Fig. 38. THE ELECTRIC ARC. lamp will sing, and no
crater will be formed ;
it is not, therefore, in such a case, necessary to distinguish
between positive and negative carbons, the current rapidly
alternating from positive to negative and back again.
THE ELECTRIC LIGHT. 49
Carbons are sold in various sizes, known by their diameter
in millimetres, and are either solid or cored, the latter containing
a core or pith, as it were, of a carbon softer than the
outside. The positive carbon should be cored, the negative
solid : the Davenport lamp is made for 8 mm. carbons. In
getting carbons for lantern purposes, the best quality, not
necessarily the highest priced, should be obtained, since
they are not an expensive item, and the regular and quiet
burning of the lamp and absence of sputtering depend
upon their quality. The Siemens and Conradty makes are
both satisfactory in this respect.
With arc lamps, what is called a resistance has to be
employed. This is generally a series of coils of platinoid or
Fig. 39. DIAGRAM OF THE CONNECTIONS FOR AN ARC LAMP.
man ganin wire, through which, as well as through the lamp,
the current passes. The size of the resistance must depend
on the pressure of the current supplied, and for this purpose
an electrician should be consulted. Fig. 39 Ehows
the way the switch and resistance should be connected
up ; it is immaterial whether the resistance is interposed
between the negative or the positive terminal and the lamp.
In the figure, the positive and negative terminals of the