Page:Scientific Memoirs, Vol. 2 (1841).djvu/80

68 time-piece); so that this mode of measuring the intensity requires less expense than any other magnetic determination. It is also very portable and convenient for travellers. The apparatus is to be placed on a table in the middle of a room, avoiding all iron in the neighbourhood; large iron rails, even at some distance, must be carefully avoided. Arrangements may also easily be made for employing it in the open air.

2. Observations to be made with this apparatus. These are of two kinds: 1. The experiments of deflection. 2. The experiments of vibration.

1. The experiments of deflection.

The measuring scale is placed horizontally, and at right angles to the magnetic meridian, with its zero point towards the east, and the needle in the centre. The small magnet bar is to be placed successively as follows:


 * 1) With its north end to the east, on the zero point of the scale: if the length of the small magnet bar is 100 millimetres, its centre will then be over the division 50mm of the scale. The needle will be deflected towards the east, and its position, $$u_0$$, is observed.
 * 2) The south end is then substituted for the north end. The needle is deflected to the west, and its position, $$u'_0$$, observed.
 * 3) The north end of the magnet bar is placed towards the east, on the division 100 millimetres. The needle is deflected easterly, and its position, $$u_1$$, observed.
 * 4) The bar is again reversed, end for end. The needle is deflected westerly, and its position, $$u'_1$$, observed.
 * 5) The north end of the magnet bar is placed towards the east, upon the division 150 millimetres. The needle is deflected easterly, and its position, $$u_2$$, observed.
 * 6) The bar is again reversed, and the needle deflected westerly, and its position, $$u'_2$$, observed.
 * 7) The north end of the magnet bar is placed towards the east, upon the division 750 millimetres. The needle is deflected easterly, and its position, $$u''_2$$, observed.
 * 8) The bar is reversed. The needle is deflected westerly, and its position, $$u'''_2$$, observed.
 * 9) The north end of the magnet is placed towards the east, on the division 800 millimetres. The needle is deflected easterly, and its position, $$u''_1$$, observed.