86 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
wards the needle whieh will at small distances cause repulsion. The needle will point, 
in the magnetic meridian, with a certain amount of force ; but as the helix is brought 
near it will point with less force, and within a certain distance will no longer point 
in the magnetic meridian, but either on one or the other side of it. There is a given 
distance within which the needle, when in the magnetic meridian, is in a position of 
unstable equilibrium, but beyond which it has a position of stable equilibrium, the 
distance varying with the strength of the exciting electric current. The power of 
the helix should be such, that when end on to the needle the latter has a position 
of stable equilibrium in the meridian. One pair of plates is quite sufficient to make 
the helix as magnetic as is needful for distances varying from 4 to 24 inches. When 
a needle is properly arranged with either a magnet or a helix to the north or south 
of it as above described, if the magnet or helix be moved west the near end of the 
needle will move east, and contrariwise. 
2971. As is well known, such a helix has a system of magnetic lines, which, passing 
through its axis, then open out and turning round on the outside re-enters again at 
the axis, the circles of magnetic force being everywhere perpendicular to the electric 
current traversing the convolutions of the helix ; and now I had, at a moment’s no- 
tice, a source of lines of magnetic power exactly of the kind required to produce, in 
association with those of the earth, a disposition of the forces coinciding either with 
those of paramagnetic or diamagnetic polarization ( 2865 . 2877 -)* 
2972. For let fig. 17 represent a section parallel to the axis of the ring-helix, 
then the two circles may represent the disposition of the magnetic force in 
that section, and the arrow-heads may serve to indicate that magnetic direc- 
tion which belongs to lines of force issuing out of the north end of a magnet. 
If such a system be suddenly produced in the midst of the earth’s lines, it 
acts upon them according to the position of the helix in relation to the direction of 
the earth’s power. Choosing the two positions in which the axis of the helix is 
parallel to the natural direction of the power, as shown by a free needle, at the place 
of observation, then two contrary effects are produced, which, as regards the lines 
exterior to the helix system, correspond to the polarity of paramagnetic and diamag- 
netic conductors. If, for instance, the helix is so placed that the polarity of its mag- 
netic lines, exterior to and in the plane of the ring, accords with that of the earth’s 
force, as in fig. 18 , then the earth’s lines are deflected as repre- 
sented, and a magnetic needle placed at a, which had taken up 
its position by the earth’s influence, will not tend to alter its 
position as the helix approaches it, though it will be acted on 
with more power. In other parts of the line, h ac,\t will alter 
its position, standing as a tangent to the curvature, and there- 
fore will be deflected sometimes one way and sometimes another, as it is carried along 
the line (or through the neighbouring lines), in place of remaining parallel to itself, 
as it would do if the electro-magnetic helix were away. 
2973. On the other hand, if the helix were turned round into the second position 
Fig. 18. 
