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 without attention.  When it is considered that open arcs for all-night burning were supplied with two pairs of carbons, the second set going into use automatically when the first were consumed, the value of the enclosed arc is apparent.  However, the open arc has served well and has given way to greater improvements.  It is rapidly disappearing from use.

The alternating-current arc-lamp was developed after the appearance of the direct-current open-arc and has been widely used.  It has no positive or negative carbons, for the alternating current is reversing in direction usually at the rate of 120 times per second; that is, it passes through 60 complete cycles during each second.  No marked craters form on the tips and the two carbons are consumed at about the same rate.  The average temperature of the carbon tips is lower than that of the positive tip of a direct-current arc, with the result that the luminous efficiency is lower.  These arcs have been made of both the open and enclosed type.  They are characterized by a humming noise due to the effect of alternating current upon the mechanism and also upon the air near the arc.  This humming sound is quite different from the occasional hissing of a direct-current arc.  When soft carbons are used, the arc is larger and apparently this mass of vapor reduces the humming considerably.  The humming is not very apparent for the enclosed alternating-current arc.  The alternating arc can easily be detected by closely observing moving objects.  If a pencil or coin be moved rapidly, a number of images appear which are due to the pulsating character of the light.  At each reversal of the current, the current reaches zero value and the arc is virtually extinguished.  Therefore, there is a maximum brightness midway between the reversals.