504 SCIENCE PROGRESS 



The second electric method of temperature measurement is 

 founded upon the fact that, in a circuit composed of two different 

 metals, if one of the junctions is heated, a thermo-electric 

 current is produced dependent on the temperature difference 

 set up. The chief advantages of the method over that of the 

 resistance thermometer are that if suitable metals are used it 

 is applicable up to considerably higher temperatures ; also that 

 it is possible by its means to measure the temperature prac- 

 tically at a point, instead of the average temperature over the 

 space occupied by the "bulb" of a resistance thermometer, 

 which may be any length up to 4 or 5 inches. The metals 

 generally employed for work at high ranges are those of the 

 platinum group, two of the most useful couples being formed 

 of a wire of pure platinum on one side against an alloy of 

 platinum with 10 per cent, of iridium or rhodium. The electro- 

 motive force given by these couples is roughly fifteen micro- 

 volts^ per degree C. for the iridium and ten for the rhodium. 

 Wires from '3 to "6 mm. diameter are usually employed. 



The rhodium couple may be used with success in technical 

 work up to about 1,300'' C, if well protected from the action of 

 furnace gases and the iridium couple to about 1,000° C. The 

 glaze of the porcelain outer tubes is, however, rapidly destroyed 

 at temperatures beyond 1200° C, and most kinds of glaze are 

 viscous at about 1,150° C. Objections have been urged that, 

 owing to the volatility of iridium, its use in a thermocouple is 

 not advisable. Experience in the use of many kinds of thermo- 

 couples has shown, however, that over the range of temperatures 

 employed for the heat treatment of iron and steel round 800° C, 

 there is little difference in the durability of the two alloys, 

 though the rhodium alloy should always be used, if continuous 

 work above 1,000° C. is intended. 



For ordinary purposes the readings of the electromotive 

 force of couples are made by connecting the terminals of the 

 couple directly to some form of galvanometer and in nearly 

 all cases the type of instrument employed is the moving-coil 

 galvanometer in which a small suspended rectangular coil, 

 carrying the whole current, moves in the field of a strong 

 permanent magnet. This type is chosen on account of its 

 freedom from external magnetic disturbance and its constancy 

 of sensitiveness. The readings are taken either by means of 



' One microvolt = one-millionth of a volt, 



