One of the most interesting improvements ever made in the arc light is that of enclosing the arc in an inner glass globe. This globe is closed airtight below with a small opening above. When the arc is formed the oxygen of the air in the inner globe is soon consumed and then combustion is no longer possible. We illustrated this by an experiment. An ordinary cork was chosen to fit the large end of an argand lamp chimney and through a hole in this was passed one of the carbon rods (Fig. 101). A metal clamp made connections between this carbon and the negative wire from the dynamo. The other carbon, attached by a clamp to the positive wire, was thrust down into the upper end of the chimney until it touched the negative carbon, and then drawn upward a short distance, drawing an arc, as we say. This soon makes an atmosphere within the chimney where combustion cannot go on for want of oxygen. The arc, however, continues to glow as in the open air, and the carbons may be drawn further apart than in the open air without breaking the arc, hence more of the external resistance may be cut out and a higher voltage put upon the lamp. [Illustration: Fig. 101] Carbons which burn out in a single night if used in open arc lamps last two weeks in enclosed arc lamps. The lower carbon, when removed from the lamp chimney of the last experiment, served as a lead pencil to write on paper. The positive carbon would not make a mark on paper. In all arc lamps carbon is distilled from the positive pencil, condensing upon the negative pencil as graphite, which is the material used in making "lead" pencils. They are called "lead" pencils because they were originally made of lead, but now they are made of graphite which is mined from the earth. As soon as the arc is broken it becomes evident that the positive carbon has been heated much the hotter of the two, a fact that could not be detected while it was lighted because of the dazzling brightness of the arc. The negative carbon turns black almost immediately, while the positive carbon remains at a bright red heat for some time. This fact needs to be borne in mind when adjusting arc light carbons in search-lights, stereopticons, and all like apparatus in which the light must be placed at the focus of a lens. That is, it is necessary to know from what point the light really comes and it is necessary to have some adjusting device to keep this point continually at the focus of the lens.