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



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. 



Ohject. — 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 ser^dce 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 ser\dce 6 



9. Puffs of smoke at end of conveyor (damper on) 



SERIES 10. 



Ohject. — 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 damper used in Series 8 and 9 seemed 

 to retard to a marked degree the propagation of explosions occur- 

 ring in the attrition miU, 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 checkiiig 

 any flame that might pass the first set of revolving paddles. Inci- 



