eliminate it; how intercept it? I see some smile at my question, and hear them exclaim: “Do not 
mariners cover with pieces of canvas or pea jackets the iron cannon in the neighborhood of the 
compass, whenever they wish to obtain exactness in their bearings?” Screens, then, ought 
effectually to furnish the means of protecting the needle from terrestrial magnetism. As to that, a 
glass sphere, inclosing the compass, would answer. 
A single word will dispel this illusion. No substance, thick or thin, has yet been discovered 
through which magnetic action, like that of gravity, does not exert its full force, without any 
abatement. The custom of covering cannon, balls, and anchors, with sails, tarred or untarred, or 
with anything else, to prevent their action on the compass, belongs to the thousand and one usages 
recorded in treatises on navigation before science had diffused its light around it. Even when 
exposed, such usages diffuse and perpetuate themselves, and are the blind powers which govern the 
world. The researches of Ampère did not absolutely require (which, in fact, would have been an 
impossibility) that his apparatus should he completely free from the attraction of the earth, it was 
sufficient that this attraction should not counteract the movement of the needle; and this simple 
reflection became the ray of light that guided the illustrious physicist, and gave rise to a kind of 
compass never before thought of. 
To understand the invention of Ampère by which a magnetic needle could be so arranged as to be 
free to obey the action of a galvanic current, and undisturbed by the magnetism of the earth, 
suppose an ordinary dipping-needle apparatus so placed that its graduated circle shall be 
perpendicular to the magnetic meridian of the place, and then so inclined to the horizon that the 
graduated circle and the needle within it shall be at right angles to the direction of the magnetic 
dip of the place where the experiment is made. In this condition the magnetism of the earth will act 
perpendicular to the direction of the needle and be opposed by the pivot on which the lower point of 
the axis rests. It will therefore be free to take any position in the plane of the divided circle 
which an extraneous force may give it, Ampère was therefore quite right in calling his new 
instrument an astatic compass. 
Ampère’s astatic needle, placed before a conducting wire, takes a direction exactly 
perpendicular to that wire, neither one second more nor less; and, a very remarkable circumstance, a 
very feeble electricity produces as much effect as a current of sufficient intensity to reduce 
metals to a state of incandescence. Here, then, is one of those simple laws that science loves to 
record, and the mind receives with confidence, and before which false theories will inevitably 
disappear. 
The discovery of Oersted reached Paris through Switzerland. At our weekly meeting on Monday, 
September 11, 1820, a member of the academy from Geneva repeated before you the experiment of the 
Danish savant. Seven days later, on the 18th of September, Ampère pre- [presented]