146 



Dr. Watson on the Determination of the 



modify the amount of the additional air carried when the bar 

 is inserted. 



Fiff. 2. 



£ 



36 



mi nil 



O 1 



2 



3 



4 



, GJO C|0 7 



c\( 



Some experiments were made to see whether the period of 

 the dummy magnet is the same when swinging in the box as 

 when swinging in the open air. These were performed by 

 taking the period with the dummy in the closed box, and 

 also with the sides of the box removed. The mean of four 

 determinations of period with the box closed was 9*4472, 

 while the mean of four determinations of period with the box 

 open gave 9 '4474. It would thus appear that at any rate 

 with a box of the size of that shown in fig, 1 the confinement 

 has no appreciable effect on the period of the magnet. 



§ 3. Comparison of the South Kensington Bars with some 

 belonging to the National Physical Laboratory. 



The National Physical Laboratory possesses a series of 

 bars by different makers obtained for the purposes of com- 

 parison, and with a view of determining the degree of ac- 

 curacy it was reasonable to expect in bars supplied with 

 magnetic apparatus sent for test. 1 was enabled, through 

 the kindness of the Director and Dr. Chree, to swing these 

 bars in my instrument, and thus compare their moments of 

 inertia with those of my bars. I had at first hoped by 

 determining the dimensions of these bars, and hence calcu- 

 lating their moments of inertia, to obtain additional data for 

 determining the moment of inertia of bar No. 10, On 



