[m 'LEOD <t BARNES] ON THE DIFi' ERENCE I N TEMPER AÏURE 75 



The small variations which we observed on the compensating and 

 thermometer leads in the first test, shown also in the traces given by 

 the other tests, indicate that t?ie slight wavy character of the various 

 records obt-rined may be due to some unavoidable variations in the 

 resistances of tbe leads. It is possible, however, to distinguish these 

 changes in the leads on the curves that we obtained and to distinguish 

 them from the more rapid, and decided changes of air temperature. 



Test of Sensitiveness. 



As we had no record of the value of the bridge wire an the record- 

 er, we were obliged to determine this by special experiment. Œihis 

 tf'L't also furnished us with a satisfactory verification of the accuracy 

 of the instrument. In order to carry out the tests, the instrument 

 was connected to a pair of 10-ohni even ratio coils, which were supplied 

 for use on the Xalder type of Carey-Foster bridge. A trace of several 

 hours was thus obtained, which was a straight line, free from any of the 

 small waves or changes which were shown in the trace of the l^ads. A 

 one-ohm sub-divided resistance box was then insertjed on one side 

 'and 2/lOths of an ohm put in the circuit. A trace of several 

 hours was obtained, similar in every way to the previous trace, but 

 separated four divisions of the scale to one side. -l/lOths of an ohm 

 were then put into the circuit and a third trace obtained, for several 

 hours. The resistances were then removed from the circuit and the 

 first trace repeated. This showed that 4/lOths of the large divisions 

 on the scale correspond to .02 ohm, the large divisions being divided 

 into 1/oths. The resistance of the thermometers was 9.264 ohms at 

 zero, which gives a change in resistance, per degree Fahren., of .020 

 olun. As two small divisions of the scale correspond exactly to .02 

 ohm, this gives a sensitiveness of two small divisions of the scale^ or 

 2/lOths of an inch, for one degree Fahren. difference between the two 

 thermometers. 



Tests of the zexo were repeated later, after an interval of three 

 months, and were found to correspond very closely. 



