of Alloys of Bismuth and Tin. DL 
By means of a fixed plane mirror the light from an illumi- 
nated scale, situated at a distance of ¥ metres from. the 
balance, was reflected to the suspended mirror, from which, 
after again undergoing reflexion at the surface of the fixed 
mirror, it was reflected into a telescope situated immediately 
above the middle of the graduated scale. 
In this way, any slight motion of the end of the pointer 
produced a rotation of the suspended mirror about a vertical 
axis, and consequently a motion of the scale across the field 
of the telescope. 
By raising the position of the centre of gravity of the 
balance the sensitiveness could be increased, until, by moving 
a rider through 1 cm., that is, by increasing the weight on 
one side of the balance by 1 mgm., a deflexion of 800 mm. 
of the telescope scale was obtained; in this position the 
balance had a period of swing of 15 seconds. 
To damp the motion of the suspended mirror, and especially 
to get rid of the rapid oscillations due to the natural period 
of the suspended system, this was provided with a set of 
vanes immersed in a vessel of paraffin-oil. The vanes were 
made of aluminium and were situated below the mirror, to 
the back of which they were rigidly attached by means of a 
metal wire. To avoid any heating of the air inside the coil 
and consequent upward currents, which would themselves 
produce an apparent diminution of the weight of the rods, a 
metal jacket was fitted into the space inside the coil, leaving 
only a cylindrical space along the axis, 10 cms. long and 
1 em. in diameter, at the upper end of the coil (fig. 1, p. 52). 
Through this jacket a stream of cold water was kept 
running “with the result that, though with a current of 
10 amperes the outside of the coil became quite hot, yet in 
the space within the water-jacket no increase of temperature 
could be detected. The current used was obtained from a 
battery of 54 storage-cells (B, fig. 1), and could he adjusted 
by means of an adjustable resistance R. 
By means of the reversing-key K, the circuit could be 
broken or the current sent in either direction through the 
coil, the magnitude of the current being indicated ‘by the 
Weston ammeter A. 
The method of carrying out an experiment was, then, as 
follows :— 
The coil was arranged in a vertical position at such a 
distance below the balance that no force was exerted on this 
when a current was passed through the coil, and the rod 
suspended vertically by means of a thread from one arm of 
the balance ; this thread passed through a hole in the case 
K 2 
