Page:Scientific Memoirs, Vol. 2 (1841).djvu/81

Rh The bar is reversed. The needle is deflected westerly, and its position, $$u'''_1$$, observed. The north end of the magnet bar is placed towards the east, on the division 900 millimetres. The needle is deflected easterly, and its position, $$u''_0$$, observed. The bar is reversed. The needle is deflected westerly, and its position, $$u'''_0$$, observed.  These twelve observations may all be completed in half an hour.

2. Experiments of vibration. The small magnet bar is to be suspended horizontally by a silk thread, to be set in vibration, and its time of vibration observed in the usual manner, which needs no further description here. The time of vibration may be determined by these experiments, with sufficient precision, in a quarter of an hour.

Taking together all the observations which are necessary for a complete measure of the absolute intensity, and allowing a quarter of an hour for arranging the apparatus and suspending the magnet bar, the experimental part of the determination can be completed in one hour. The observer may give his determination greater certainty and accuracy by repetition.

The following observations made with this instrument at Göttingen are given as an example.

Göttingen, January 18, 1837. 1. Experiments of deflection.

In these experiments, the distance, $$R$$, of the centre of the small magnet bar from the centre of the compass, was successively,