- 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:
- A small compass needle.
- A small magnet bar, which may be suspended to a silk thread, and vibrated.
- 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[1]. 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, 17½ in 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 dimensions 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 draw the suspension thread through the eye of the needle; it is better to make the small bar precisely 100 millimetres 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 millimetres.
This simple apparatus is sufficient for the absolute measure of the magnetic intensity. It is furnished by M. Meyerstein, of Göttingen, 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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