nted an arc-lamp and an efficient form of
dynamo to supply the electrical energy. The first arc-lamps were
ordinary direct-current open arcs and the carbons were made from
high-grade coke, lampblack, and syrup. The upper positive carbon in
these lamps is consumed at a rate of one to two inches per hour.
Inasmuch as about 85 per cent. of the total light is emitted by the
upper (positive) carbon and most of this from the crater, the lower
carbon is made as small as possible in order not to obstruct any more
light than necessary. The positive carbon of the open arc is often cored
and the negative is a smaller one of solid carbon. This combination
operates quite satisfactorily, but sometimes solid carbons are used
outdoors. The voltage across the arc is about 50 volts.

In 1846 Staite discovered that the carbons of an arc enclosed in a glass
vessel into which the air was not freely admitted were consumed less
rapidly than when the arc operated in the open air. After the
appearance of the dynamo, when increased attention was given to the
development of arc-lamps, this principle of enclosing the arcs was again
considered. The early attempts in about 1880 were unsuccessful because
low voltages were used and it was not until the discovery was made that
the negative tip builds up considerably for voltages under 65 volts,
that higher voltages were employed. In 1893 marked improvements were
consummated and Jandus brought out a successful enclosed arc operating
at 80 volts. Marks contributed largely to the success of the enclosed
arc by showing that a small current and a high voltage of 80 to 85 volts
were the requisites for a satisfactory enclosed arc.

The principle of the enclosed arc is simple. A closely fitting glass
globe surrounds the arc, the fit being as close as the feeding of the
carbons will permit. When the arc is struck the oxygen is rapidly
consumed and the heated gases and the enclosure check the supply of
fresh air. The result is that the carbons are consumed about one tenth
as rapidly as in the open arc. There is no crater formed on the positive
tip and the arc wanders considerably. The efficiency of the enclosed arc
as a light-producer is lower than that of the open arc, but it found
favor because of its slow rate of consumption of the carbons and
consequent decreased attention necessary. This arc operates a hundred
hours or more without trimming, and will therefore operate a week or
more in street-lighting with