12 BULLETIN 379^ U. S. DEPARTMENT OF AGRTCULTUEE. 



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 ah 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, therebj^ destroying any difference of potential existing 

 within the machine. This common wire was thoroughl}^ grounded 

 to relieve the machine of an}^ 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 ol' 

 electricity in these cases, as was found in the other systems of wiring. 



BLOWER SYSTEMS. . 



The possibilitj^ of removing the dust from the vicinity of the 

 cylinder by means of a fan or blower located on top of the separator 



