Page:Electricity (1912) Kapp.djvu/142

138 has a very large negative temperature coefficient, and in consequence the exaggeration as regards changes in current strength mentioned on the previous page in connection with carbon lamps, is much greater; in fact, it is so great that the working becomes unstable if the pencil be used alone, even on a circuit of perfectly constant voltage. To make the use of such a pencil possible, it is necessary to protect it against excess of current and consequent disintegration. This is done by correcting its negative temperature coefficient by the addition of a conductor having a large positive temperature coefficient. Such a conductor is iron when near the point of red heat. The pencil and this additional resistance, termed technically a "ballast resistance" are arranged tandem-fashion, or, as it is called, "in series," so that the current first passes through the ballast resistance and then through the pencil. The object of the ballast resistance is to keep the current as near constant as possible; and this object is attained by the fact that, owing to the peculiar property of hot iron to very largely increase its resistance for even a slight increase of temperature, the e.m.f. absorbed by the ballast resistance becomes large even with a small