GEAIX-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 during 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 penny attached by sealing wax to a J-inch 
glass tube 10 inches long. The copper lead and proof plane each gave 
good deflections of the electroscope when these devices were applied 
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 hi 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 slight 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 lead 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 
hi 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 
machine is dissipated so rapidly by leakage, which is largely due to 
