72 Prof, Challis on a Theory of Magnetic Force. 
12. The directive action of terrestrial magnetism on galvanic 
currents admits of the following explanation. Let the current 
flow in a circular electrode moveable about a vertical axis, and 
let the electrode be so placed in the plane of the magnetic 
meridian, that the direction of the current at the lowest part may 
be from north to south. A diameter to the circle being drawn 
in the direction of the terrestrial current, that is, from south 
towards north in the line of the dipping needle, at and near the 
south extremity of the diameter this current will be cenfluent 
with the circular movement about the electrode on the easé side, 
and opposed to it on the west side. Hence on the latter side 
there will be an excess of pressure which will tend to move the 
electrode eastward. At the other end of the diameter the 
terrestrial current flows with the circular stream on the west side 
and opposes it on the east side, and consequently tends to move 
the electrode westward. Hence on the whole the electrode will 
rotate in the direction which accords with observation, till it 
takes a position perpendicular to the plane of the meridian. 
13. The directive action of terrestrial magnetism on the 
dipping needle is explained as follows. When the needle is in its 
normal position, the extremity A directed northward, the secon- 
dary streams flow longitudinally in the same direction as the 
primary terrestrial stream, and their transverse motions are 
perpendicular to the latter. There is consequently no tendency 
of the needle to move from this position. Now let the axis of 
the magnet be inclined to its normal position in any given 
direction. The secondary streams will not be altered by this 
change, because, as before explained, the accelerative forces to 
which they owe their origin are independent of the direction of 
the primary stream. But in the new position the two streams 
will influence each other so as to give rise to dynamic action on 
the needle. Conceive, for the sake of distinctness, the end A of 
the needle to deviate about 30° from the normal position towards 
the west. Then resolving both the primary and the secondary 
streams in the directions perpendicular and parallel to the needle, 
the respective perpendicular streams along the north portion of 
the needle (this being the portion from which the secondary 
streams zssue) will be opposed to each other on the west side of 
the needle, and conspire on the east side. Thus the needle will 
be urged eastward. Onthe south portion of the needle, at which 
the secondary streams are confluent, the perpendicular compo- 
nents are opposed to each other on the eas¢ side and conspire on 
the west side, and that portion is consequently urged westward. 
As the two actions are equal and opposite, the needle has simply 
a motion of rotation, and the directive action is always éowards the 
normal position. 
14, It follows that the directive force of the earth’s magnetism 
