180 APPLIED SCIENCE 



in a space between a soft iron core and the pole of a perma- 

 nent magnet. A light tubular pointer, attached to the coil 

 moves over a graduated scale. The current is introduced 

 into the coil by means of two spiral springs which serve to 

 control the movement of the pointer. When a current 

 passes through the wire, the coil tends to turn in a certain 

 direction against the action of the springs which tend to hold 

 it in place. The amount of deflection is proportional to the 

 voltage. The scale is graduated to read in volts. It is 

 accurate and substantial. This instrument should not be 

 placed in a strong field, as such a field will permanently 

 affect the permanent magnet. In this case the scale must 

 be graduated again. 



215. Electric Pyrometers. In certain manufacturing 

 processes it is necessary to determine the temperature of 

 furnaces. Hence the need of some instrument that is simple, 



accurate, and capable of 

 being handled by a work- 

 3) man without special me- 

 chanical or . electrical 

 knowledge. Such an in- 



Fio. 81. Pyrometer. & 



strument is found in the 



electric pyrometer (Fig. 81) which consists of a thermo-ele- 

 ment, or insertion tube, for exposure to the heat, and a 

 sensitive galvanometer to indicate the temperature at a con- 

 venient distance from the source of heat. 



The principle underlying this pyrometer is that when any 

 metal is heated an electric current is set up. The intensity 

 of the current depends upon the temperature to which the 

 metal is heated. Thus, measuring the current measures also 

 the temperature at the extremities of the metal. 



