•11069. 



in Terms of the Mechanical Equivalent of Heat. 27 



two results obtain for the same point and time. The value of 



(VI is 0-18567. 



The measurements of V and V were made in the same man- 

 ner as those of I and b. 



The results are V = 263'91 cm, b' = 2*07 cm, for most experi- 

 ments. A correction was applied when V varied. Hence 



. — v — v- 6 i^r 



Each of the angles, a and <p, is the mean of eight readings 

 taken to 1-. The former was about 13°, the latter 6°. T was 

 obtained by observing ten transits with a seconds clock, allow- 

 ing the coil to vibrate for several minutes and then taking ten 

 more transits. The difference between the mean times of the 

 two series divided by the number of vibrations gives T very 

 exactly. The difference between the values before and after 

 the experiment never exceeded 1 part in 3000. The mean 

 value is about 2*42 seconds. H was determined before and 

 after the main experiment. 



The quantities in the formula for J remaining to be dis- 

 cussed are t, h and the deflections. To treat these intelligibly 

 I proceed to describe the method of experiment exactly. 



First, a determination of H was made. The calorimeter was 

 then weighed, filled with distilled water at a temperature usually 

 2° or 3° below that of the air, carefully wiped with a towel 

 to remove moisture, again weighed and placed in the water- 

 jacket. Its amalgamated electrodes were placed in the mercury- 

 cups with the terminals of the two circuits, the main circuit 

 being broken at the commutator. The water-jacket was kept 

 permanently filled and stood in a room of fairly constant tem- 

 perature so that its temperature changed little during the experi- 

 ment. The thermometer was placed in position and the stirrer 

 started. During a few minutes readings were taken of the 

 thermometer and of three auxiliary thermometers, giving the 

 temper.-itures of the jacket, the 30,000 ohm coil and the air 

 near the stem of the principal thermometer, the time of each 

 reading being noted by a seconds clock. The circuit was then 

 closed and a galvanometer reading taken, one of the commuta- 

 tors was reversed and another reading taken, the time of each 

 reading being noted. The time of passage of the mercury of 

 the thermometer over several successive scale-divisions was then 

 taken, also readings of the other thermometers. Two more 

 commutator reversals and galvanometer readings followed, then 

 another set of thermometer readings, and this alternation was 

 continued for about 40 minutes, during which time the ther- 

 mometer rose about 12° C. Usually sixteen galvanometer 

 readings were taken and seven groups of thermometer read- 

 ings comprising 35 or 40 readings of the principal thermome- 



