490 



Prof. W. A. Douglas Rudge on the 



shown to be true in the case of the iron dust-cloud. As 

 indicated in Table II. , iron acquires a negative charge, and 

 the air should consequently be positive. In order to show 

 this, the wire-gauze cylinder was placed between the poles 

 of a strong electromagnet, and an insulated brass tube made 

 to surround the end portion. A cloud of iron dust was 

 raised in the usual manner, but most of the iron particles 

 were retained by che magnet, and the current of air im- 

 pinging upon the outer tube gave it a distinct positive charge 

 which could be detected by removing it from the neighbour- 

 hood of the inner cylinder. 



In order to be certain that the charges were actually 

 attached to the dusts, and to the air, the method of con- 

 ducting the experiments was altered. It might be considered 

 probable that the charges shown by the metal cylinder were 

 due to the friction of the particles upon the insulated wire 

 gauze, but this is not the case, for if so, the charge upon the 

 cylinders would be opposite to that upon the dust. 



A second insulated wire gauze cylinder was arranged 

 around the first (fig. 2), and on raising a cloud in the inner 

 cylinder the dust was carried through the meshes of the gauze,, 



1%. 2. 



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and reached the outer cylinder. The charge indicated by loth 

 cylinders was the same. This was not due to an induction 

 effect, for on removing the inner cylinder the charge persisted 

 upon the outer one. A third cylinder surrounding the second 

 one also acquired a charge similar to that on the innermost 

 one, so that it seems to be quite conclusive that the charge 

 observed is actually attached to the dust particles. 



The air is also carried through the cylinders with the dust, 

 so that the charge acquired by the outer cylinder will be the 

 sum of the charges given up by the dust and air, and these 

 charges would of course be opposite in sign. It is possible. 



