106 CLAYS AND CLAY INDUSTRY. 



trial piece. If the cones are heated too rapidly, especially those 

 containing a large percentage of iron, they are apt to blister. 1 



Thermoelectric pyrometer. — This pyrometer, which is the only 

 one that will be described in this report, is one of the best in- 

 struments for measuring temperatures. It is based on the prin- 

 ciple of generating an electric current by the heating of a ther- 

 mopile or thermoelectric couple. This consists of two wires, one 

 of platinum and the other of an alloy of 90 per cent, platinum 

 and 10 per cent rhodium. These two are fastened together at 

 one end, while the two free ends are carried to a galvanometer 

 which measures the intensity of the current. That portion of 

 the wires which is inserted into the furnace or kiln is placed within 

 two fire-clay tubes, one of the latter being smaller and sliding 

 within the other in order to- insulate the wires from each other. 

 The larger tube has a closed end to protect the wires from the 

 action of the fire gases. 



To measure the temperature of a furnace or kiln the tube 

 containing the wires is placed in it either before starting the 

 fire, or else during the burning. If the latter method is adopted, 

 the tube must be introduced very slowly to- prevent its being 

 cracked by sudden heating. The degrees of temperature are 

 measured by the amount of deflection of the needle of the gal- 

 vanometer. 



Thermoelectric pyrometers are useful for measuring the rate 

 at which the temperature of a kiln is rising, or for detecting 

 fluctuations in the same. It is not necessary to place the galvano- 

 meter near the kiln, for it can be kept in the office some rods 

 away. This pyrometer is not to be used as a substitute for 

 Seger cones but to supplement them. The more modern forms 

 have an automatic recording device. As at present put on the 

 market, the thermoelectric pyrometer costs about $180, and the 

 price, delicacy of the instrument, and lack of realization of its 

 importance have all tended to restrict its use. However, many 

 of the larger clay-working plants are adopting it, as it is better 

 than other forms of pyrometer for general use and probably 

 more accurate. It can be used up to 1600 C. (29 12 F.) 



1 In this connection see paper by A. F. Hottinger, Trans. Amer. Ceramic 

 Society, Vol. Ill, p. 180. 



