GAUSS AND WEBER OX TERRESTRIAL, MAGNETISM, fi7 



6. The advantages, in point of accuracy, of the dimensions 

 adopted in the apparatus. 



1. The parts of the small apparatus. 



In addition to a clock or chronometer, this apparatus consists 

 of three parts : 



1 . A small compass needle. 



2. A small magnet bar, which may be suspended to a silk 

 thread, and vibrated. 



3. A measuring scale 1 metre in length. 



The needle of the compass from which the present description 

 is taken was 60 millimetres in length, and the arc was divided 

 to whole degrees only. In order that so small a compass should 

 lead to useful results, it is necessary that the observer should 

 be able to estimate with certainty the 10th part of a degree*. 

 The needle may be somewhat larger; but the reasons, which 

 render it advisable that it should never exceed 100 millimetres, 

 will be given at the conclusion of this chapter. 



The small magnet bar was 101 millimetres in length, 174 "i 

 breadth, and 142 grammes in weight ; it may be vibrated by 

 suspending it to a silk thread bound crossways round the 

 middle of the bar. It is advantageous that it should be made 

 an exact parallelopiped, in order that, its m eight and dimen- 

 sions being known, its moment of inertia may be calculated. 

 It may also be provided at the middle with a small hole, through 

 which a sewing needle may be passed, in which case it is merely 

 necessary to di'aw the suspension tlu-ead through the eye of the 

 needle ; it is better to make the small bar precisely 100 millime- 

 tres in length. 



The breadth of the measuring scale must be such as to allow 

 of the compass being placed on its centre ; this scale need only 

 be divided to 50 milUmetres. 



This simple apparatus is sufficient for the absolute measure of 

 the magnetic intensity. It is furnished by M. Meyerstein, of 

 Gottingen, for 9 dollars and a half, (of course, exclusive of the 



* This estimation, which under other circumstances is easy to accomplish, 

 presents in this case some difficulty, arising from the point of the magnetic 

 needle being usually at a little distance from the divided arc : to get over this 

 difficulty the following method is adopted: a mirror is laid horizontally on the 

 table before the needle, and the eye, before it reads off the position of the 

 needle, must be so placed that the prolongation of the needle would bisect the 

 reflected image of the eye. 



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