ASSOCIATION UNIT OF RESISTANCE IN ABSOLUTE MEASURE. 0*33 
special experiment, in which the ring (with wire circuit open) was oscillated backwards 
and forwards through a small angle in time with the natural vibrations of the magnet,, 
allowed us to verify the plane of the currents. A marked effect was produced when the 
plane of the ring was east and west, but nothing could be detected with certainty when it 
was north and south—the opposite of what would happen with the wire circuit closed. 
After this, no doubt could remain but that most of the disturbance was due to currents 
in the ring, and subsequent spinnings after the removal of the wire have proved that no 
sensible part of it was caused by leakage through the silk insulation. The existence of 
this disturbance, however, so far modified our original plan as to induce us to omit the 
second ring as giving rise to too great a complication. 
The suspended magnet was made of four pieces of steel attached to the edges of a cube 
of pith and of such length (about inch) as to be equivalent in their action to an 
infinitely small magnet at the centre of the cube. Before the pieces were put together 
the approximate equality of their magnetic moments was ascertained. The resultant 
moment was between six and seven times as great as that used in our former experi¬ 
ments. In virtue of the greater radius of the coil, this important advantage was 
obtained without undue increase of the correction for magnetic moment, which 
amounted to about ‘004, only twice as great as before. The effect of mechanical 
disturbances, such as air currents, was still further reduced by diminishing the size of 
the mirror, particularly in its horizontal dimension. On both accounts the influence 
of air currents was probably lessened about 15 times, and, in fact, no marked disturbance 
was now caused by the proximity of a lamp to the magnet box.'' 5 ' In consequence of 
these changes, however, it was found necessary to introduce an inertia ring in order to 
bring the time of vibration up to the amount (about 5|- seconds from rest to rest) neces¬ 
sary for convenient observation. The diameter of the ring was about § inch, and the 
whole weight of the suspended parts was not too great to be borne easily by a single 
fibre of silk. A brass wire passing between the spokes of the ring jjrevented the needle 
from making a complete revolution. 
The enlarged scale of the apparatus allowed us to introduce a great improvement 
into the arrangement of the case necessary for screening the suspended parts from the 
mechanical disturbance of the air caused by the revolution of the coil. A bra,ss tube 
of an inch in diameter was not too large to pass freely through the hollow axis. At 
its lower extremity (fig. 1) it was provided with an outside screw, to which the magnet 
box was attached air-tight. By unscrewing the box, whose a,perture was large enough 
to allow the inertia ring to pass, the suspended parts could be exposed to view, and by 
drawing up the brass tube they could be removed altogether, so as to allow the coil to 
be dismounted, without breaking the fibre. The upper end of the fibre was attached 
to a brass rod sliding in a socket at the upper end of the tube, by which the height of 
the magnet could readily be adjusted. The whole was supported on three screws 
MDCCCLXXXIL 
* See pp. 115-132 of the former paper. 
4 Q 
