142 



Dr. Watson on the Determination of the 



carry with it during its motion. Since, however, the inertia- 

 bar itself will carry some additional air, if we assume that 

 the inertia of the bar is that deduced from its dimensions, we 

 shall underestimate the inertia of the magnet and the air 

 which it carries. 



To investigate this subject an exact copy of the magnet 

 belonging to magnetometer No. 61 (Elliott Bros.) was made 

 in brass, the moment of inertia of this dummy being almost 

 exactly the same as that of the magnet, and this dummy 

 magnet was swung in the instrument used for comparing the 

 moments of inertia of the bars. 



Three suspension-fibres of different diameters were used, 

 that of intermediate thickness being chosen so that the 

 period of the dummy was nearly the same as that of the magnet 

 when swinging in the earth's field in England. The period 

 of the dummy was determined both with and without the 

 inertia-bar No. 1, and the results are collected together in the 

 following table. 



Dummy alone. 

 I eriod. 



2-5085 

 2-5084 

 2-5085 



Temperature. 



o 

 15-6 

 16-3 

 14-4 



Pressure, 

 cms. 



. 77-3 

 751 



74-7 



Logarithmic 

 Decrement. 



•0014 



•0012 



•0012 



Means... 2-5085 



15-4 



15-4 

 16-4 

 16-5 



75-7 



5-8 



5-7 

 5-5 



•0013 



2-5074 



2-5071 

 2-5072 



•0006 

 •0006 



Means... 2-5072 



16-1 



5-7 



•0006 



Dummy+Bae No. 1. 









4-3527 



4-3528 

 4-3528 

 4-3527 



16-5 

 16-5 

 15-8 

 15-9 



75-8 

 75-8 

 75-9 

 75-9 



•0012 

 •0011 



Means... 4-3527 5 



16-2 



75-8 5 



•0012 



4-3511 

 4-3510 

 4-3512 

 4-3510 

 4-3510 



15-7 

 157 

 15-7 

 15-4 

 15-4 



60 



5-8 

 5-9 

 63 

 6-0 



•0007 

 •0007 



Means... 43511 



15-6 



60 



•0007 



Dummy alone. 

 9-4418 

 9-4419 

 94419 

 94419 



18-2 

 18-2 

 17-2 

 17-3 



75-6 

 75-6 

 75-4 

 75-4 



•0028 

 •0029 



Means... 9-4419 



17-7 



75-5 



■0028 



