44 BULLETIN 681, U. S. DEPARTMENT OF AGRICULTURE. 



the relatively high atmospheric humidity of this region, that no poten- 

 tial is built up in the frame of the machine. 



2. Should an excessive amount of static electricity be developed 

 in a mill of an}^ kind, proper methods of gi'ounding the machine ^ 

 will go far toward eliminating any danger that may be due to sparks 

 from static charges. 



STATIC ELECTRICITY AS CAUSE OF EXPLOSIONS. 



In 25 attempts to ignite inflammable dusts with the spark from 

 an induction coil, not a single explosion was obtained (Series 6, 7, and 

 12). From the fact that an electric arc would produce explosions 

 under identical conditions, it would seem that the production of arn 

 explosion is largely dependent upon the -extent as well as the intensity 

 of the source of ignition, especially when there is a high velocity of 

 the dust past the igniter, as is the case m the attrition mill. The 

 temperature of the electric arc is estimated at about 6,200° F., but 

 laboratory experiments ^ have shown that certain dusts can be 

 ignited at as low a temperature as 1,100° F. with a source of heat 

 having a comparatively small area. Consequently, great care should 

 be exercised to eliminate every possible source of ignition. 



Conclusions : 



1. It is comparatively difficult to ignite infiammable dusts in an 

 attrition miU by means of static electricity. 



,2. Nevertheless, every precaution should be taken to eliminate 

 static electricity in the operation of any kind of mill. 



EFFECT OF ATMOSPHERIC HUMIDITY. 



The effect of humidity, both relative and absolute, upon the inflam- 

 mability of dusts is not yet known, and can not be defiiiitely deter- 

 mined except by laboratory investigations under carefully controlled 

 conditions. In the tests in the attrition mill the humidity of the 

 atmosphere was always determined by means of a carefully cali- 

 brated sling psychrometer. The relative humidity ranged from 44 

 per cent to very nearly saturation, or 100 per cent, in the case of a 

 steady rain. No deductions could be made as to the effect of the 

 relative humidity on the inflammability of grain dusts, for too many 

 variable factors were involved. It is, however, of interest to note 

 that in Series 3, durmg a steady ram, the regularity of explosions 

 happened to be greater than in any other series. This fact seems to 

 indicate, at least, that a high relative humidity does not tend to 

 decrease the inflammability of dusts, except in so far as the moisture 

 content of the materials fed into the mill may be considerably 

 increased after a sufficient length of time. 



1 References 3S and 39, Bibliography. 2 Reference 26, Bibliograpliy. 



