CAPACITY FOR HEAT OF METALS AT DIFFERENT TEMPERATURES. 143 



and the relay circuit had a pair of aluminium plates in water as shunts, to diminish 

 sparking. 



A considerahle fraction of the heat necessary to maintain the tank at the required 

 temperature was given by a constant supply, while the relay operated the fine 

 adjustment. 



The intermittent lamps were placed close to the stirrer, and thus the whole 

 arrangement tended to keep the oscillations of the temperature within narrow limits. 

 At some temperatures we had thermometers by which we could detect changes 

 (' -.00 C., but when the apparatus was working satisfactorily we at no time observed 

 oscillations of this magnitude. 



Another circumstance which perhaps assisted in diminishing the oscillations was 

 the fact that the stirring was sufficiently vigorous to cause a continual vibration of 

 the U-tube of the thermostat and so prevent any adhesion of the mercury to the 

 platinum point which established connection with the relay. 



SKCTION X. 

 (1) The Total Heat Method. 



The metal under examination was cooled to a temperature lower than that of the 

 tank (0 ) and the fall being observed by means of bridge-wire olraervations, it was 

 stopped when it had passed below the range of the bridge. 



The contact-maker was then set at a certain reading, which, for clearness, we will 

 specify as 9.* Meanwhile, the "heating" current was adjusted on an auxiliary 

 coil enclosed in a tube containing oil. This auxiliary coil was a duplicate of the 

 coil in the metal block and the change-over from the one to the other could be 

 effected by the depression of a recording key. Before the transference of the current, 

 the temperature of the block rose very slowly by radiation, &c., and could be followed 

 by the gradual approach of the galvanometer spot to its zero mark. 



The rate of rise was of the order of 0'0 4 3G Pt per second, consequently the 

 temperature throughout the block was practically uniform. 



The moment the temperature had reached the bridge reading 9, as indicated by 

 the transit of the spot across its zero mark, the heating current was switched over, 

 the keyt at the same time recording the time on the chronograph tape. A slight 

 readjustment of the rheostat was usually required to maintain exact potential balance 

 when the change-over was effected. The contact-maker of the bridge was then set 

 at the next integer, 8 (the temperature interval from 9 to 8 being roughly 



* This was the customary starting point. 



t The key was so constructed that any time lag between the marking of the tape and the actual switch 

 on was compensated for during the operation of switching off. 



