Magnetical Experiments, $6\ 43 
ments recently made, the following is a description. It consists 
of a small table of brass, A, Plate I, Fig. 1, 4£ inches square, and 
3§ inches high, having a plate of brass C attached to it by 
hinges, and moveable by means of the wheel and pinion D, E, 
through an arch of 250 degrees of a vertical circle. This plate 
has a small straight groove running from end to end, in the line 
a a , for the purpose of receiving bars of metal, the polarity of 
which is to be determined. These bars are readily fixed to the 
plate, by being slipped through a circular aperture in the end 
of a spring b, which, perforating the moveable plate, and acting 
downward, firmly embraces any substance laid along the groove. 
The angular position of the moyeable plate is marked by a circle 
FF, screwed upon the side of the table. It is graduated so as 
to mark the angle between the moveable plate and the horizontal 
plane, whether above or below it. To insure accuracy in this 
angle, the true horizontal position of the table, and with it the 
horizontal line on the circle, is determined by means of a spirit- 
level G; and that the movements of the plane may be accurate, 
and the angle marked true, the pin which passes through the 
hinges also forms the centre of the wheel D, and terminates ex- 
actly in the centre of the graduated circle F. H is a moveable 
flat plate of brass, divided into rhumbs and degrees : it is fur- 
nished with a magnetic needle, having an agate cap traversing 
on a brass or steel point. The needle can be changed according 
to the nature of the circumstances ; a very lightly and strongly 
magnetized one being used in delicate experiments. The com- 
pass or plate carrying the needle, being moveable, its distance 
from the bar resting on the limb C, may be varied at pleasure. 
The centre of the hinges is one-tenth of an inch above the level 
of the table ; the magnetized needle stands at the same elevation ; 
and the bars in use being one-fourth of an inch diameter, are 
sunk in the groove of the moveable plate to such a depth, that 
their axis, or centre, precisely corresponds with the centre of the 
hinges ; hence the middle of the extremity of each bar is at the 
same elevation, and at the same distance from the needle in 
every position of the moveable limb. To give firmness to the 
instrument in making experiments, the table is fixed by the feet 
to a mass of lead I, of seven or eight pounds weight. By means 
of this plate of lead, which has a screw d at each corner, the 
whole apparatus is readily put into a horizontal position. As 
