8 BULLETIN 343, U. S. DEPARTMENT OF AGRICULTURE. 
in the field circuit direct current could be generated at any desired 
voltage from 100 to 700 volts. The direct-current motor connected 
to the grinder was also separately excited and the speed of rotation 
controlled by regulating the voltage applied to the armature, the 
voltage control being brought about by variation of the field current 
of the direct-current generator. The speed of the grinder motor 
could be varied from 100 revolutions per minute to 300 revolutions 
per minute, the capacity with overload at the same time varying 
from approximately 340 horsepower to 750 horsepower. 
The efficiency of the direct-current motor could be determined very 
readily by means of curves showing the stray power and heat losses 
at different speeds and current loads and knowledge of the power 
supplied to the motor. The latter data were obtained by means of 
accurately calibrated indicating and recording electrical instruments. 
The various pieces of apparatus used in the manufacture of the 
pulp were driven by individual direct-connected motors. The appa- 
ratus for wood preparation, 40-inch swing saw, and “‘Green Bay” 
barker, together with the wet machine vacuum pump, were driven 
from a single-belted motor. Some of the individual motors were of 
the variable-speed type, and as a result the pieces of apparatus to 
which they were connected could be driven at the most effective speeds. 
PULP MACHINERY AND AUXILIARY EQUIPMENT. 
All of the pieces of machinery installed in the Wausau laboratory 
were of commercial size and design and were loaned either by manu- 
facturers or others interested in the work being carried on. 
The grinder was built by the Friction Pulley and Machine Works. 
It took a stone 54 inches in diameter by 27-inch face and had three 
14-inch cylinders. Each cylinder was provided with a pressure gauge 
and the water was supplied by two triplex pumps. The pressure at 
which the water was pumped was regulated by relief valves; pressures 
as high as 120 pounds per square inch could be obtained. 
A recording thermometer gave a record of the temperature in the 
grinder pit. From the grinder pit the pulp was passed through a 
mechanically agitated sliver screen, then pumped to a storage tank 
by means of a 5-inch centrifugal pump, and from there pumped to a 
centrifugal screen. A variable-speed motor direct connected to the 
screen made it possible to obtain speeds of rotation from 400 to 600 
revolutions per minute. Throughout the tests, however, the speed 
was maintained at 500 revolutions per minute. The plate in the 
centrifugal screen was perforated with holes 0.065 inch in diameter. 
The tailings from the centrifugal screen were rescreened through a 
12-plate Harmon diaphragm screen, the plates used being the Union 
Screen Plate Co.’s type B, cut with 0.012-inch slots. 
