.14 BULLETIN 681, U. S. DEPARTMENT OF AGEICULTUEE. 
the relatively high atmospheric hurnidit}' 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 any kind, proper methods of grounding the machine 1 
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 an 
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 in the attrition mill. The 
temperature of the electric arc is estimated at about 6,200° F., but 
laboratory experiments 2 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 inflamniable dusts in an 
attrition mill 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 definitely 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 gram dusts, for too many 
variable factors were involved. It is, however, of interest to note 
that in Series 3, during a stead}- ram, the regularity of explosions 
happened to be greater than in an}^ 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. 
i References 38 and 39, Bibliography. 2 Reference 26, Bibliography. 
