of Iron and German Silver, 



491 



meter were introduced into the oil, and then the aperture was 

 closed with cotton wool. In this way an extreme retardation 

 of the cooling of the oil was attained (in 45 minutes it amounted, 

 on the average, to about 5° C), and the temperature of oil, 

 points, and thermometer could at any moment be taken as equal. 

 A second thermometer was suspended by the side of the bar, 

 giving the temperature of the surrounding air, and consequently 

 that of the bar. During the experiments the temperature of the 

 air remained nearly constant. The two thermometers were read 

 oflf with the aid of a telescope. It is now easy, from simulta- 

 neous observation of the temperatures of the oil and the bar, 

 and of the accompanying deliection of the needle, to deduce, 

 according to equation (11), the value of R. For this a single 

 simultaneous observation would indeed have sufficed; yet, for 

 control, several such observations were always made. The de- 

 termination of R in the first series of experiments with the iron 

 rod may serve as an example. 



Deflection 

 of the 

 needle. 



Tempera- 

 ture of the 

 oil. 



Tempera- 

 ture of the 

 middle of 

 the rod. 



Difference. 



R. 



Calculated 

 difference. 



5 19-3 

 4 22-2 

 3 24-5 



29 16 C. 

 24 70 „ 

 20 32 „ 



4-85 C. 

 4-80 „ 

 4-78 „ 



24-31 

 19-90 

 1554 



259-5 

 259-4 

 2603 



24-32 C. 



19-92 „ 

 15-50 „ 



From this we obtain the mean value 

 R = 259-7. 



U = 259-7 



tan <j) 



1-0-63 smc/)-^ 



(12) 



The numbers in the sixth column are obtained by calculating 

 the differences of temperature from the observed angles 0. In 

 those experiments in which the ends of the iron rod were alter- 

 nately heated and cooled, the points A (fig. 5) possessed the 

 lower, the middle of the rod the higher temperature, and, ac- 

 cordingly, the current was in the opposite direction to that in 

 the above experiments. With the aid of the regulator, however, 

 it was easy to make the detiection of the needle follow the same 

 direction. Besides, no particular stress need be laid upon the 

 deflection being in the same direction, since before the experi- 

 ments the turns of the multiplier-coil were placed parallel to the 

 plane of the magnetic meridian, and the defiections, when a con- 

 stant current passed through the multiplier in either direction, 

 were equally great on both sides of the position of rest. 



