CAPACITY FOR HEAT OF METALS AT DIFFERENT TEMPERATURES. 141 



Densiti**. 



The densities of the metuls were found by weighing in air and water, the usual 

 precautions being observed. The values were checked by calculation from the 

 dimensions and weight. 



SECTION VIII. 

 Measurement of Time, 



The only records made during the course of an experiment were the times of transit 

 of the temperature of the thermometer in the block past bridge-wire divisions, and as 

 these were effected mechanically by the depression of a key, the observer's attention 

 could be concentrated on the transits. 



The time-recording arrangements may be briefly described as follows : 



An electrically driven seconds pendulum bob, suspended by an Invar rod, at each 

 swing tilted over an exhausted tube about 2 inches long by -inch diameter, fixed in 

 a frame capable of oscillating about an axis perpendicular to the length of the tube. 



As the carriage bearing the tube was unstable about this axis, a slight impulse 

 sent it over from one stop to the other, causing a small mercury pellet to run down 

 the tube and make momentary connection between two platinum wires fused midway 

 into it, thus completing the electro-magnet circuit in the chronograph. 



A series of equally spaced dots about 2 cm. apart on the tape indicated seconds, 

 while the marks of the respective observer's keys were recorded on opposite sides of 

 the tape. By counting and measuring the fraction, the times of transit could be 



obtained to . 2 \^ second. 



Although the seconds pendulum kept a fairly constant rate being fitted with a 

 cut-out device to keep its amplitude to a definite limit the absolute rate was 

 determined for each experiment by comparison with a rated chronometer.* 



This comparison of the total time also afforded a check on the accuracy of the 

 reading of the tape, always a somewhat laborious process, the lengths of tape used 

 varying from 20 to 60 metres. 



SECTION IX. 

 Temperature Control of tfie Huthn. 



The absolute steadiness of the bath temperature was of prime importance, as our 

 conclusions were based on the assumption that the temperature of the walls 

 surrounding the blocks remained constant throughout an experiment. 



Wo are indebted to Mr. T. J. Williams, 63, Bute Street, Docks, Cardiff, for the loan of this 

 chronometer, and for kindly checking its rate from time to time. 



