250 Mr Douglas Rudge, A dust electrical machine. 
O is very strongly charged, readily giving a potential gradient 
in a room of several hundred volts per metre. This charge upon 
the air is readily detected by aid of an insulated wire tipped 
with a radioactive substance, the wire rapidly taking the potential — 
of the air in its neighbourhood, and if discharged will quickly 
become charged again. The air in a room may remain charged 
for more than half an hour after the dust has settled, so that it 
may be concluded that the air itself is actually charged. 
A 
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SIL 
The charge acquired by the apparatus has probably a two- — 
fold origin (1) that due to the raising of the dust, and (2) that — 
due to friction of the dust particles against the walls of the tube. 
At first 1t appeared as though only one charge was present upon 
the dust but recent experiments have shown that both kinds of 
electricity are present, one of them however predominating. 
Further experiments are now in progress. 
