Page:Electricity (1912) Kapp.djvu/140

136 the furnace, the higher becomes the resistance of the spiral, so that by a suitable graduation of the scale of the instrument, this may be used to show what temperature actually exists in the furnace.

The influence of temperature on the resistance of a material is a physical attribute of the material, such as its specific resistance itself, or, for the matter of that, as all its physical and chemical properties. We express this particular property by saying that the material has such and such a "temperature coefficient." Thus copper has a temperature coefficient of + 0.0038, meaning that the resistance increases by 0.38 per cent, for every degree of temperature increase. The + sign means that the coefficient is positive, that is, refers to an increase, not a decrease of resistance. There are, however, certain substances which have a negative temperature coefficient. In these materials the resistance diminishes as they get hotter. Most liquid conductors have this property, and of solid conductors carbon is a familiar example. The resistance of a carbon filament incandescent lamp is greater when the lamp is cold than when it is alight. In this case the heat is generated by the current passing through