PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 25 
to the same current. It was necessary to study the deportment of both of these bars 
separately. 
A helix was formed of covered copper wire one-twentieth of an inch thick: the 
space within the helix was rectangular, and was 1 inch long, 0‘7 inch high, and 1 inch 
wide : the external diameter of the helix 
was 3 inches. Within the rectangular 
space the body to be examined was 
suspended by a fibre which descended 
through a slit in the helix. The latter 
was placed between the two flat poles of 
an electro-magnet, and could thus be 
caused to act upon the bar within it, 
either alone or in combination with the 
magnet. The disposition will be at once 
understood from fig. 6, which gives a front view of the arrangement. 
Action of Magnet alone : division of Bars into Normal and Abnor mal. — A bar of soft 
iron suspended in the magnetic field will set its longest dimension from pole to pole : 
this is the normal deportment of paramagnetic bodies. A bar of bismuth, whose 
planes of principal cleavage are throughout parallel to its length, suspended in the 
magnetic field with the said planes vertical, will set its longest dimension at right 
angles to the line joining the poles : this is the normal deportment of diamagnetic 
bodies. We will therefore, for the sake of distinction, call the former a normal para- 
magnetic bar, and the latter a normal diamagnetic bar. 
A bar of compressed carbonate of iron dust, whose shortest dimension coincides 
with the line of pressure, will, when suspended in the magnetic field with the said 
line horizontal, set its length equatorial. A bar of compressed bismuth dust, simi- 
larly suspended, or a bar of bismuth whose principal planes of crystallization are 
transverse to its length, will set its length axial in the magnetic field. We will call 
the former of these an abnormal paramagnetic bar, and the latter an abnormal dia- 
magnetic bar. 
Action of Current alone on normal and abnormal bars. — A normal paramagnetic bar 
was suspended in the helix above described ; when a current was sent through the 
latter, the bar set its longest horizontal dimension parallel to the axis of the helix, 
and consequently perpendicular to the coils. 
An abnormal paramagnetic bar was suspended in the same manner; when a cur- 
rent was sent through the helix, the bar set its longest dimension perpendicular to the 
axis of the helix, and consequently parallel to the coils. 
A normal diamagnetic bar was delicately suspended in the same helix ; on the 
passage of the current it acted precisely as the abnormal magnetic bar ; setting its 
longest dimension perpendicular to the axis of the helix and parallel to the coils. 
MDCCCLV. E 
Fig. 6. 
