184 Trans. Acad. Sci. of St. Louis. 
A cross section through one of the suspended masses is 
shown in Fig. 2. 
Fig. 2. 
This outside metal shield was not air-tight. It was 
formed of sheets of flexible tinned iron, the parts of which 
overlapped. They were tied in place by windings of 
twine. It was considered an advantage to allow convec- 
tion currents which might form in the layer of air be- 
tween the two sheets of metal forming the sides of the 
enclosing case some opportunity to escape into the outer 
air, 
The large masses M were thus separated from the sus- 
pended masses m’ by two superposed sheets of cardboard 
and a sheet of metal, which were clamped and sealed to 
the wood frame of Fig. 1, a layer of air about % inch 
in thickness, and the outer sheets of metal, forming part 
of the metal shield enclosing the entire device. 
Both of the masses M , and the shield around the sus- 
pended masses, were insulated as before described. The 
large masses and the shield around the suspended masses 
were connected by means of large copper wires. Be- 
tween the masses M, and the shield were sheets of glass, 
not shown in Fig. 2. 
In the work to be described, the air around the large 
masses and screen was electrified by a noiseless discharge 
from 800 pin points which were mounted in strips of 
metal hung upon insulated metal rods three feet from the 
