Page:Electricity (1912) Kapp.djvu/139

Rh unless provision has been made in the construction of the machine for such application of thermometers, this may not be done. It is in this connection that the influence of temperature on the resistance of copper comes in very useful. We need only measure the resistance of the copper coils before the machine is set to work, that is to say, whilst it is at ordinary room temperature, and repeat the measurement after the machine has got hot through working. The increase of resistance thus found may be used to calculate the rise of temperature in the interior of the machine. According to the best modern practice, this rise should not exceed about 50° C.

Another important application of the fact that all metals increase their resistance with a rising temperature is made in the so-called "electric pyrometer," an instrument for measuring the very high temperatures in metallurgical furnaces. Essentially, the pyrometer consists of a porcelain tube containing a spiral of platinum wire, which is put into the furnace. The spiral is joined to other wires of low resistance, which lead to some kind of measuring instrument, indicating the resistance of the platinum spiral. The hotter