Fig. 100
It may be pettifogging to say that the upper portion of this iron rod is the core of the magnetic field, and its lower portion is the armature. Yet this is right, and pettifogging may be right when it is the only way to bring out the fact.
Our great study now is to produce light without heat, or at least to come as near to it as the firefly does. The firefly gives 98 per cent. light and two per cent. heat. The arc lamp gives 12 per cent. light and 88 per cent. heat. The carbon filament gives 4 per cent. light and 96 per cent. heat. When we have made considerable progress in that direction we shall take electric lamps out of the chapter on electric heating and form a new chapter on electric lighting.
One might expect that a rod made of carbon would quickly burn up, particularly when raised to the exceeding high temperature of the electric arc. While it is true that carbon in the form of charcoal burns so readily that it is used instead of kindlings for lighting a fire, carbon in the form of graphite in our so-called "lead" pencils and carbon as it is prepared for electric light pencils burns only very slowly even at exceedingly high temperatures. The carbon rods used in arc lamps endure a temperature of over 6000 degrees, without losing more than one inch an hour, and half of that is simply volatilized—not burned.
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.
Fig. 101