12 BULLETIN 379, U. S. DEPARTMENT OF AGRICULTURE. 
static electricity was responsible for the smut dust ignition., The 
method usually adopted was to connect a No. 12 or 14 weatherproof 
copper wire to the journal box of the cylinder shaft and to an iron. 
rod driven into the ground, and to make similar connection with the 
bearing of the stacker fan. 
The arrangement was identical with the grounding device installed 
in connection with the dextrine reel (already referred to on p. 11) 
in an eastern plant where an explosion occurred. The film of oil 
existing between a shaft and its bearing may, and in the opinion of 
many does, act as an insulation, and thus render the method just 
described inefficient. When this was explained to the men in the field, 
many of them decided not to depend upon grounding from the bear- 
ings but to install a properly grounded direct copper brush contact 
to each shaft as recommended by the investigators. 
In a number of cases the owners were desirous of having the 
investigators equip their machines to conform to the ideas ad- 
vanced, and about 15 machines were so wired. It is of interest to 
note that no explosion or fire occurred in any machine grounded in 
this manner, while in one case an outfit not so equipped had two fires. 
Second method.—The second method was the one which the inves- 
tigators generally advocated, and was readily accepted by the me- 
chanics and experienced thresher men, for it provides a double 
security. All moving parts were connected with each other and to a 
common wire, thereby destroying any difference of potential existing 
within the machine. This common wire was thoroughly grounded 
to relieve the machine of any surplus charge that might accumulate 
in it. About 10 machines were wired in this manner, consisting of a 
direct brush contact to the revolving shafts by means of a stiff strip 
or bar of copper. These machines, although located in parts of the 
territory where explosions and fires were most frequent, had neither 
an explosion nor a fire after being so equipped. 
Third method.—The third method, that of grounding the moving 
parts on some heavy metallic part of the frame, was based on the 
theory that if the difference of potential between parts could be 
destroyed there would be no tendency for sparking between moving 
shafts and other metal parts. Several machines were wired in this 
manner during the season and all finished threshing without ex- 
periencing any fires or explosions. Field tests with electrical meas- 
uring instruments, however, did not show the complete absence ot 
electricity in these cases, as was found in the other systems of wiring. 
BLOWER SYSTEMS. 
The possibility of removing the dust from the vicinity of the 
cylinder by means of a fan or blower located on top of the separator 
