AMPERE'S EXPERIMENT. 235 



is to say, a magnetic system indifferent to the 

 action of the terrestrial globe; then causing a 

 fixed current to act on it, placed horizontally in a 

 direction perpendicular to the magnetic meridian, 

 from east to west, he saw that the action of this 

 current was precisely t{ie same as the action of 

 the earth." ' 



At rest, the gyroscope is a grouping of mole- 

 cules, or a molecular arrangement. Pre-existing 

 electric currents are present; but, moving in all 

 directions, they neutralize each other. In order 

 to bring the gyroscope into an electro-dynamic 

 condition, a diamagnetic current must be induced 

 around it sufficiently intense to overpower the 

 terrestrial currents. This induction can be ef- 

 fected by whirling the disc, like a humming top, 

 by a string wound round its axis, in which case 

 the impulse will soon be exhausted ; or by an elec- 

 tric current, when a relative continuity of action 

 can be obtained, better illustrating the effect of 

 the terrestrial currents. With -the battery an arti- 

 ficial current changer is used. 



" The law controlling the movements of the 

 gyroscope is as follows: Where a body is acted 

 upon by two systems of forces, tending to produce 

 rotations about two separate axes lying in the same 

 plane, the resultant motion will be rotation about 

 a new axis situated in the same plane between the 

 directions of the other two. It is coincident with 



1 Guillemin's Forces of Nature, p. 612. 



