414 Descripiion and Use of the Apparatus employed 
box H, and to which is fixed a small steel axis, which rests 
upon the bevelled edge of a plate of iron N, in which mercury 
is put in contact with the axis. The part FG of the wire 
passes into the tube of glass, and is soldered to the copper-box 
G, to which is attached a small steel axis similar to the other, 
and resting similarly on an iron plate M. The plates M and N 
are supported by the feet PQ and RS, which communicate with 
the mercury in the boxwood cups T, U into which are plunged 
the two conductors from the two ends of the pile. A thin 
wooden lozenge ZV, supports the rectangular wire at Z and V, 
in order to keep it from bending. 
In order to observe the effects of the directive action of an 
electric current upon a loadstone, without its being affected by 
different causes, M. Ampere constructed a needlei, which he calls 
the Astatic Needle. This instrument, represented in Fig. 8. 
consists of a magnetic needle AB, fixed perpendicularly to an 
axis CD, which can be placed in any direction by the screws 
and pinions at E, F. The needle thus placed, can only move by 
turning in a plane perpendicular to this axis, in which its centre 
of gravity is exactly placed, so that we may be sure, before it is 
magnetised, that its gravity has no tendency to make it change 
its position. It is then rendered magnetic, and the instrument 
serves to prove, that when the plane in which the needle moves 
is not perpendicular to the direction of the dipping needle, the 
earth’s magnetism tends to make the needle take the direction of 
the lines traced upon its plane, which are the nearest possible to 
the axis of the dipping needle, that is to say, the direction of 
the projection of this axis on the same plane. By means of the 
screws E, F, the plane in which the needle moves is made per- 
pendicular to the direction of the dipping needle, and the earth’s 
magnetism has then no power to direct it, and it -becomes com- 
pletely astatic. The instrument carries in the same plane a 
graduated circle LMN, on which are fixed two small bars of 
glass GH, IK, for attaching the conducting wires, whose direct- 
ing power will act alone, and without complication, along with 
gravity and terrestrial magnetism. , 
The principal experiment which is made with this apparatus, 
is to shew that the angle between the directions of the needle 
