This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
By an open arc is meant an arc lamp in which the arc is exposed to the atmosphere, while in the enclosed arc an inner or enclosing globe surrounds the arc, and this globe is covered with a cap which renders it nearly air-tight. Fig. 38 is a good example of an enclosed arc as manufactured by the General Electric Company.
Fig. 3S. Enclosed Arc Lamp with Carbon-Feed Mechanism.