GKAIN-DUST EXPLOSIONS. 15 
SERIES 5. 
Object. — This test was run as a demonstration to Government 
officials from the Bureau of Mines (G. A. Hulett and J. K. Clement) 
and from the Bureau of Chemistry (D. J. Price, H. H. Brown, and W. 
G. Goodenow). 
Conditions. — The arc was located near the disks, which were run at 
their normal speed. Various kinds of grain products (except ele- 
vator dust, the supply of which had been exhausted) were fed into 
the attrition mill. 
Results. — Although explosions were readily obtained with flour and 
wheat scourings, none was obtained with corn, as in Series 3. In 
fact, the regularity and force of the explosions, none of which caused 
the flame to extend as far as the end (E) (PI. Ill, fig. 1) of the conveyor, 
were not nearly as great as in Series 3, in spite of the relatively low 
humidity (49 per cent) of Series 5, as compared with other tests. 
SERIES 6 AND 7. 
Object. — These experiments were run to determine whether an 
explosion would result from the discharge of static electricity. 
Conditions. — The conditions of testing were similar to those of 
previous tests with the electric arc. The carbons were replaced by 
f-inch brass balls screwed on the ends of long J-inch iron rods, which 
replaced the f-inch iron carbon holders. These j-inch rods were in- 
serted through porcelains in the wooden door of the hood, to insure 
insulation from the metal frame of the machine, and were connected 
by 15-foot leads to the poles of a small Wimshurst machine. 
Results. — Under normal conditions this machine was capable of 
producing 1 a good spark about 3 inches long, but under the condi- 
tions of the test, with the long leads, and great leakage, it gave only 
a very weak spark, hardly more than \ inch long, between the brass 
knobs near the disks of the mill. It was, therefore, decided to make 
use of an induction coil instead of the static machine. By this 
means a very hot and almost continuous spark 2 inches long could 
be obtained between the brass knobs. On two different days a total 
of 20 attempts was made to obtain explosions with various inflam- 
mable dusts by means of the spark from an induction coil. The 
spark gap was varied from \ to If niches. No explosions occurred. 
This negative result, of course, does not indicate that static electricity 
can not cause an explosion in an attrition mill, but rather that un- 
der the particular conditions of the test, the mixtures of air and dust 
were not sufficiently inflammable to be ignited by the sparks from an 
induction coil. In order to show that the electric arc, on account of 
the greater amount and intensity of heat produced, could ignite 
the same mixtures under the same conditions, the carbons were 
quickly substituted for the brass knobs, and then within 10 minutes 
