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 



