the Roberts-Austen Recording Pyrometer. 69 



of a slip of mica. The free ends of the wires were soldered 

 to the leads from the recorder, and these " cold junctions " 

 were kept in ice, or in cold water the temperature of winch 

 was noted. More recently an hypsometer has been used, 

 as shown at M in the figure, thus keeping the " cold junctions" 

 al (he temperature of boiling water, which has the advantn^c 

 of being more constant than the temperature at which cold 

 water can readily be maintained ; the hypsometer also requires 

 less attention than a " cold junction " involving the use of ice. 

 The interval of temperature to be measured is also reduced, 

 and errors in the E.M.F. are decreased in the same pro- 

 portion. 



Method of taking a Record. — The photographic plate is first 

 placed on the float F, using a red light in the room, and a 

 cover is placed over the slit to keep out the spot of light. 

 The galvanometer is short-circuited by the shunt-box e, and 

 the thermo-couple, in its clay tube, is placed in the metal or 

 alloy, N, which has been melted in a gas-furnace. The flame is 

 then either turned out or reduced in size, so that the furnace 

 may cool slowly. The plugs P, P' are placed in position in 

 the potentiometer, and the shunts are gradually removed from 

 the galvanometer as the balance is obtained. When the 

 galvanometer has its full sensibility, the spot of light travels 

 almost across the photographic plate every time that the 

 position of P / is changed in order to maintain the balance as 

 the furnace cools down. This change is effected by placing 

 another plug immediately to the left of F f before the latter is 

 removed ; when it becomes necessary to move the plug P the 

 galvanometer is short-circuited. When the metal has cooled 

 to the point at which a record is desired, the water-clock is 

 started, and the cover removed from the slit to admit the spot 

 of light from the galvanometer to the photographic plate. 

 As the movement of the spot of light can be seen on a scale 

 immediately above the slit, it is easy to follow the progress of 

 the experiment without interfering with the record. As the 

 metal cools down and the photographic plate rises, successive 

 tracks of the spot of light may be obtained on the plate, each 

 track corresponding to a particular position of the plugs. 

 When the cooling has been carried sufficiently far, the 

 galvanometer is disconnected from the thermo-couple and 

 short-circuited by a contact-piece at /, and a datum-line is 

 run on the plate to indicate the zero position of the galvano- 

 meter. 



It is also necessary 



(1) To take the barometer-reading to determine the tem- 

 perature of the " cold junction." 



