Page:Scientific Memoirs, Vol. 2 (1841).djvu/96

84 Milan, with the magnetometer of that place, in October, 1836, viz. 2·01839.

To gain a clear idea of the Import of these numbers, the determination and application of which have been hitherto under consideration, imagine a number of small steel bars, perfectly alike, and each weighing about 2 grammes, or of an ounce. Imagine further a balance, of which the length of the arms bears to 1 metre the same proportion that 1 metre bears to the space of descent in 1 second (204 millimetres nearly); suppose one of these steel bars to be attached in a parallel direction to the horizontal beam of the balance, in such manner that the equilibrium is not thereby disturbed. Then render all the steel bars (including the one attached to the balance) equally magnetic, and to such a degree that when another of their number is placed vertically beneath the scale at the distance of 1 metre from the attached magnet bar, th of a milligramme must be placed in the scale to preserve equilibrium. When the magnetism of all the bars has been regulated in this manner, place one of the bars horizontally, and at right angles to a small compass needle, 1 metre from the centre of the needle beneath, taking care that as the compass needle is deflected from the magnetic meridian, the bar be also turned so that they may preserve their rectangular position. Lastly, calculate how many such bars are required that their united force may deflect the compass needle 90°; the number of bars gives the terrestrial magnetism in thousandths of its absolute measure.

We may conceive in like manner the number which represents the absolute measure of the terrestrial magnetism to represent the number of these bars reckoned in thousands, the forces of which must be united to cause, at a distance of a metre, a deviation of 90°. This would require at

6. On the Advantages of the Dimensions selected for the small Measuring Apparatus.

Before concluding this article, we have to discuss the accuracy of which the absolute measurement of intensity with the apparatus described is susceptible, and on what it is founded. It has been already remarked, that the absolute intensity can be