ATMOSPHERIC MAGNETISM — DEFLECTION OF THE LINES OF FORCE. 
87 
(2972.), then the effect upon the direction of the neighbouring lines of force would 
be as in fig. 19. Needles placed at d and e would again be deflected 
from the natural position given to them by the earth, but they 
would be deflected in a contrary direction to that which would 
be taken if they were in corresponding situations under the 
former arrangement. This figure and state of things represents 
the paramagnetic disposition of the forces, as the former did the diamagnetic con- 
dition. 
2974. It is not pretended that the whole of these arrangements of forces are like 
those of the cases of paramagnetic and diamagnetic conductors. Independent systems 
are here introduced into the midst of the earth’s magnetic power, and the central part 
of each arrangement must therefore be excepted ; there are also attractions inwards 
and repulsions outwards, when the needle is at a and f, which do not take place in the 
cases of mere magnetic conduction. But external to these helix systems, the arrange- 
ment imposed upon the lines of force from the earth is in accordance with that pro- 
duced by diamagnetic and paramagnetic conductors, and at distances from 2 inches 
to 2 or 3 feet ; the lines of force thus altered, and those contorted by the sun and atmo- 
sphere in the great field of nature, are comparable in their direction, and may be con- 
sidered as representing each other. 
2975. In order to obtain a simple result of the action of such a centre of force on 
the magnetic lines of the earth, I adjusted a rod in the direction of the dipping-needle, 
and also a plane at the foot of it parallel to the magnetic equator at London. Then 
suspending a small magnet, half an inch long, from cocoon-silk, so that when hanging 
it should be parallel to the magnetic equator, it was adjusted so as to be near to the 
plane at the foot of the rod representing dip. The ring-helix (2970.) was then asso- 
ciated with the voltaic pair, so that contact could be completed at any moment, and 
being always retained parallel to itself and to the plane of the magnetic equator, 
could be brought into the vicinity of the needle on all sides, above or below, and its 
action upon it observed. As the object was to represent the sun’s action, the current 
v^as so sent through the helix that its upper face would repel the north end of a mag- 
netic needle ; for then a magnet, outside of and in the plane of the ring, would not 
tend to have its position changed, and the disposition of the forces of the earth under 
the influence of the helix was as in fig. 18, or like that of a diamagnetic conductor. 
2976. In making observations of this kind, and especially if the ring-helix is pur- 
posely retained at a considerable distance from tbe needle, it is better not to connect 
the helix permanently with the battery and then carry it towards and by the needle, 
but rather to choose the place where the helix action is to be observed, and when the 
helix is there to make contact with the battery ; the motion and direction of the 
needle is then easily observed ; or if it still, through reason of distance, be feeble, 
making and breaking contact a few times isochrcnously with the vibrations of the 
needle soon raises the effect to any degree required. 
