16 BULLETIX 681, U. S. DEPARTMENT OF AGEICULTUEE. 
of Series 7, a number of explosions was obtained in Series 8 with the 
same substances by means of the electric arc. 
SERIES 8 AND 9. 
Object. — The object of these tests was to note the effect of a single 
revolving damper in preventing the propagation of explosions 
occurring in an attrition mill. 
Conditions. — The revolving damper was installed as shown in 
figure 2. The details of this damper may be seen in figure 3, which 
shows two such dampers in series. The by-pass and butterfly 
valve were inserted to make certain that the flame would be propa- 
gated that far when the damper was off, or out of service. 
Results. — ■ 
1. Total number of attempts to obtain explosions with arc. 73 
2. Total number of explosions 57 
3. Damper in service during explosion 29 
4. Damper out of sendee during explosion 28 
5. Flashes of flame in by-pass (damper off) 11 
6. Flashes of flame at end of conveyor (damper on or off) 
7. Puffs of smoke at end of conveyor (damper off) 21 
8. Flashes of flame past damper when in sendee 6 
9. Puffs of smoke at end of conveyor (damper on) 
SERIES 10. 
Object. — To determine whether static electricity was built up 
during the operation of an attrition mill. 
Conditions. — For this purpose a sensitive gold-leaf electroscope was 
procured from the Department of Physics. The collecting devices con- 
sisted of a comb made up of pins and tin foil located near one of the 
driving belts and of two 5-foot flexible lamp cords with 3-inch pieces 
of No. 14 bare copper wire soldered to each end. During the tests 
the casing of the electroscope was grounded by being held in the 
hand, while the binding part of the gold leaf was connected by the 
flexible wire lead to the collecting comb, or any part where it was 
desired to test for static electricity. The attrition mill was run 
at its normal speed cf 2,200 revolutions per minute. 
Results. — See Table 10, page 29. 
SERIES 11. 
Object. — To note the effect of a double damper in preventing the 
propagation of explosions. 
Conditions. — The single clamper used in Series 8 and 9 seemed 
to retard to a marked degree the propagation of explosions occur- 
ring in the attrition mill, but did not always prevent the flame 
from getting past the damper. Consequently, a double damper 
(figs. 3 and 4 and PI. IV) was installed, with a view to checking 
any flame that might pass the first set of revolving paddles. Inci- 
