576 WEBER ON A TRANSPORTABLE MAGNETOMETER. 



Absolute Measure of the Intensitij. 

 The measurement of the intensity divides itself into four 

 parts. 1. The detex-mination of torsion. 2. Of the moment 

 of inertia of the deflecting bar. 3. The experiments of de- 

 flection. 4. The experiments of vibration. I will confine my- 

 self in this place, for the sake of brevity, to two parts, viz. 

 the determination of the moment of inertia, and the experiments 

 of deflection, which are especially instructive towards a know- 

 ledge of the instrument. The determination of torsion has 

 been already spoken of in the measurement of declination, and 

 the experiments of vibration are so simple and so well known, 

 that it is sufficient to give their results. 



1. Determination of the Moment of Inertia. 

 The deflecting bar is suspended to a thread or wire, and is 

 then vibrated : 1) without a weight; 2) with a weight, the mo- 

 ment of inertia of which is known. 



Hence the mean time of vibration without a weight is = 6"'696, 

 and with a weight = 12"*039. For determining the moment of 

 inertia of the weight we have the following data : 1) the length I 

 of the deflecting bar, or the distance apart of the threads which 

 hang from its two ends and support two equal cylindrical weights j 

 2) the mass 2 j5 ; 3) the radius r of the two cylinders. 



1 = 93«"»i-42 



2 J5 = 50000"»g 



r = 4™™-60 



