88 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVII.) 
2977- There are certain positions in respect of the needle as a centre which must 
be clearly comprehended. The magnetic axis is a line through the centre of the free 
regular needle parallel to the direction of the earth’s lines of force, whatever that 
may be, at the place where the experiments may be made. The magnetic equator 
plane is a plane passing through the centre of the needle perpendicular to the mag- 
netic axis. The plane of the magnetic meridian is that plane which coincides with 
the magnetic axis, and also with the direction in which the declination needle points. 
This position always occurs with the magnets that are employed for observation, being 
a consequence of the method in which they are supported ; it would not be taken by 
a needle placed at right angles on its mechanical axis, the latter being in the mag- 
netic axis. 
2978 . When the ring-helix, situated as before explained ( 2975 .), was anywhere in 
the plane of the magnetic meridian, it exerted no action on the declination needle 
tending to change its position. When the helix was anywhere in the plane of the 
magnetic equator, it exerted no action on the needle to make it change its direction. 
These are the only places in which the helix does not affect the position of the 
needle. 
2979 . These two planes of no variation divide the space around the magnet into four 
quadrants, and the helix being in any one of these, affects the needle, altering its 
declination. The deflection of the line of force for two neighbouring quadrants is in 
the contrary direction, so that as the helix passes from the neutral line into one or 
the other quadrant, the declination of the needle changes. 
2980 . If the helix be above or below the magnetic equator and be carried round 
the magnetic axis travelling along a line of latitude, then the needle makes one large 
oscillation to the right, and another to the left during the circuit. Supposing that 
the experiment commences with the helix above the equator, and in the plane of the 
magnetic meridian north of the needle, if it then proceeds by west to south and on 
by east to its original position the north end of the needle will first go westward ; 
will then stop and return eastward, passing the mean position, and will finally return 
westward and settle in its first or original direction. All the time the helix is to 
magnetic east of the needle it will cause the same deflection, and also as long as it is 
in the west ; the deflection will be more or less, but not change in direction as 
regards the neutral place. The position of the helix north or south of the needle 
is of no consequence as to the direction of the declination, provided it remain on 
the same side of the magnetic meridian, though it is to the amount. If the helix be 
below the magnetic equator the direction of the declination is reversed, but then 
again it does not change whilst the helix remains east or west of the needle and its 
plane of mean declination. 
2981 . If we carry the helix round the needle in a plane perpendicular to the planes 
of the magnetic equator and meridian, so as to traverse in succession the four qua- 
drants, then the needle makes two to and fro vibrations (instead of one) during the 
