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XY. The Determination of the Moment of Inertia of the 

 Magnets used in the Measurement of the Horizontal Com- 

 ponent of the Earth's Field. By W. Watson, A.R.C.S., 

 D.Sc.j F.RM.. Assistant Professor of Physics at the Royal 

 College of Science, London *. 



ONE of the constants required when determining the 

 horizontal component of the earth's magnetic field by 

 the ordinary method, is the moment of inertia of the magnet 

 which is used in the vibration experiment. Nearly all the 

 magnetometers which are used in English-speaking lands are 

 tested at the Kew Observatory ; and the custom there has 

 been, I believe, to determine the moment of inertia of the 

 cylindrical brass bar supplied with each instrument by calcu- 

 lation from its dimensions, then by measuring the period of 

 the magnet alone and when loaded with this bar to calculate 

 the moment of inertia of the magnet. This method pre- 

 supposes that the density of the inertia-bar is uniform 

 throughout. Now it is by no means easy to secure a bar of 

 which the density is uniform throughout, and further it is 

 difficult to test whether such uniformity has been secured. 



This question as to the uniformity of the inertia-bars supplied 

 with magnetometers has been brought into some prominence 

 lately; for the differences at first obtained when comparing 

 the magnetometers intended for the Indian Magnetic Survey 

 with the Kew standards were finally traced to want of 

 uniformity in the inertia-bars. 



It seems to me that more reliable and uniform results 

 would be obtained if a somewhat different procedure were 

 adopted. Namely, to determine once for all, with very 

 great care, the moment of inertia of a standard bar, and then 

 to determine the moment of inertia of the bars supplied with 

 the different magnetometers sent to be tested by comparing 

 them experimentally with the standard bar. Even if the 

 standard adopted were slightly wrong, so that the values 

 given for the bars belonging to the different instruments were 

 uniformly either too high or too low, the confusion caused 

 w T ould be much less than where, as at present, the values 

 obtained may, owing to want of uniformity in the bars, be 

 sometimes too high and sometimes too low. 



In the following paper is described an instrument suitable 

 for comparing the moment of inertia of bars, together with 

 some experiments made with a view to determining the 

 moment of inertia of a standard bar, and an investigation on 

 the influence of the air on the period. 



* Communicated by the Physical Society : read April 14, 1905. 



