GRAIN-DUST EXPLOSIONS. 43 



STATIC ELECTRICITY IN ATTRITION MILLS. 



Repeated tests were made on various days, June 22, 23, 26, and 

 July 1, 1916, to detect any charges of static electricity that might 

 have been built up durmg the operation of the attrition mill. The 

 devices employed for transferring the charge to the electroscope in- 

 cluded the copper wire described in Series 10 (p. 16), and also a 

 proof plane, consisting of a peiniy attached by seahng wax to a ^-inch 

 glass tube 10 inches long. The copper lead and proof plane each gave 

 good deflections of the electroscope when these devices were apphed 

 to bodies known to have charges of static electricity, and therefore 

 it was certain that the methods employed for indicating the presence 

 of static electricity were reliable. In the tests involving the use of 

 the proof plane, it was held near the moving belt, or brought in con- 

 tact with the frame of the machine, and then touched to the binding 

 post of the electroscope, which was grounded by holding in the hand. 



None of the tests run on the summer days mentioned, in none of 

 which the humidity was lower than 70 per cent, indicated the slightest 

 trace of static electricity during the operation of the attrition mill, 

 with and without the feeding of various grains. This result indicated 

 either that no static charge was being generated or the machine was 

 too well grounded, either directly or by leakage, to retain any charge 

 that might have been generated. 



By means of the usual voltmeter method the metal frame of the 

 attrition mill was found to be well insulated as far as low voltages 

 (110) were concerned. 



In order to test qualitatively the insulation and leakage for high 

 voltages, one terminal of a Wimshurst machine was connected to 

 the frame of the attrition mill. By means of the proof plane and 

 electroscope it was found that the attrition mill was capable of 

 retaining a sUght charge for a few seconds after the charging wire 

 had been removed. The indications of the very sensitive electro- 

 scope, however, were so feeble as to liead to the conclusion that, 

 while the attrition mill was fairly well insulated, its leakage (owing 

 to the many corners and edges, covering of dust, and also atmos- 

 pheric moisture) and its capacity were so great that a considerable 

 quantity of electricity would be required to establish a high potential 

 on the frame. 



Further tests (Series 10) on a winter day (temperature 42° and 

 humidity 54 per cent) indicated the development of static electricity 

 in the driving belts, due probably to the comparatively low moisture 

 content of the air, but not in the frame of the machine. 



Safe conclusions from these tests are as follows : 



1. Any static electricity that may be generated in this particular 

 maohine is dissipated so rapidly by leakage, which is largely due to 



