ME. G-. GO EE ON ELECTEOTOESION. 
od i 
alike. The magnitudes of the torsions in each case appeared to be proportional to the 
length of iron within the helix. 
14. Interference of terrestrial magnetism. 
It must not be forgotten that in all these experiments terrestrial magnetism operates, 
and more or less influences the results ; also that the iron is liable to have its residual 
magnetism weakened by the repeated passage of an axial current in either direction 
through it ; and if the lower end of it is a north pole, the polarity is often reversed. 
A single contact of the battery is sufficient to remove the polarity, if the current is 
powerful and the iron wire a small one. 
In consequence of not being able to maintain a bar or wire of iron perfectly free 
from longitudinal magnetism during the passage of an axial current through it whilst 
in a vertical position, and the difficulty of detecting minute torsions with the bar in a 
horizontal one, I was unable to ascertain if an axial current alone would produce torsion, 
by experimenting with a demagnetized iron bar at right angles to the magnetic meri- 
dian. But as the magnitudes of the torsional effects of an axial current increase with 
the strength of the longitudinal magnetic polarity of the iron, and opposite longitu- 
dinal magnetic polarities enable each axial current to produce opposite torsions, it is 
highly probable that if a vertical iron rod or wire could be maintained in as perfectly 
an unmagnetized state in a longitudinal direction as it can in a transverse one, an axial 
current alone, like a coil one alone (see p. 533), would produce little or no torsion. 
15. Will axial currents remove the residuary effect of coil ones 1 
Some special experiments were made with the thin wrought-iron tube of 10 mm. dia- 
meter to determine the above question, or whether the previous application of a coil- 
current would enable any number of torsions in alternately opposite directions to be 
obtained by continually reversing the axial stream. The current from 12 cells arranged 
as 6 was employed, and no weight was attached to the tube. 
1. With a south pole below . — Any number of torsions could be obtained by such 
means, and the longitudinal magnetic polarity of the iron was not entirely removed by 
many passages of the current, because terrestrial magnetism continually renewed it, and 
because the axial current was small in relation to the conducting-power of the tube, 
and therefore produced less disturbing effect upon the longitudinal magnetism. 
2. With a north pole below . — The first two downward currents, and then two upward 
ones, produced movements in the normal direction, the last of these being only deficient 
in magnitude ; but all the succeeding currents caused torsions agreeing in direction with 
the existence of a south pole below, and the longitudinally magnetic polarity, tested by 
means of a small compass-needle, was found reversed after several axial currents had 
passed. 
4 c 
MDCCCLXXIV. 
