312 



Mr. W. Hibbert on a 



Date. 



Temperature 

 of Inductor. 



Lines in Inductor Field. 



April 16, 1891 



;; 22 ;; :::::: 



„ 23 „ 



May 8 „ 



„ 23 , 



„ 27 ;, 



„ 30 „ 



June 6 ,, 



„ 12 „ 



„ 29 „ 



July 10 „ 



„ 27 „ 



„ 31 „ 



Sept. 22 „ 



Nov. 10 „ 



50 C. 



20 



12-5 



13-5 



16-5 



13 



13-5 



16 



18 



22 



21-5 



19 



20 



17-5 



16 



13 



No I. 

 22,030 

 21,790 

 21,730 

 21,710 

 21,710 

 21,680 



21,720 

 21,720 

 21,780 

 21,720 

 21,790 

 21,700 

 21,780 

 21,690 

 21,700 



No. II. 



32,360 

 32,420 

 32,330 



32,4ib 

 32,380 

 32,470 

 32,345 

 32,510 

 32,470 

 32,460 

 32,400 

 32,400 



No. III. 



29,140 

 29,290 

 29,270 

 29,260 

 29,290 

 29,500 

 29,550 

 29,530 

 29,470 

 29,480 



Density of lines in 1 

 air-gap per sq. cm. J 





515 



770 



700 



In discussing the figures it must be remembered that high 

 accuracy was not attainable at the time of working. The ob- 

 servations were all made in comparatively brief intervals of 

 leisure, and it is likely that the probable error is about 1 in 

 300 or 400. 



There is practically no evidence of magnetic decay in 

 seven months. Such small changes as are indicated point the 

 other way, but I am inclined to attribute most of them to 

 slight inaccuracy of the temperature-correction for the re- 

 sistances of the circuit. 



The two intruments that were reduced by a demagnetizing 

 current (Nos. II. and III.) show a tendency to rise. This 

 tendency is most marked in the instrument which was re- 

 duced most. It is evident that only a slight reduction is to 

 be allowed. 



No. L, which was magnetized warm, but not " reduced," 

 fell while cooling, and then showed a very slight tendency to 

 increase. It is better than the other two, and is constant 

 enough to be used for any purpose for which an earth inductor 

 is employed. 



Nothing is here said of the temperature variation of this 

 magnetic field*. I have made some observations on that 



* Magnetic field is of course not the right name. The instrument 

 raises once again the question of naming the quantity sometimes called 

 " total field," "total induction," or so many "gausses." 



