10 BULLETIN 1179, U. S. DEPARTMENT OF AGRICULTURE. 
ages varying from 40 to 100 in steps of 20 and also halves of these 
voltages were obtained. A view of this first furnace is shown in 
Plate I. 
Practically complete volatilization of phosphoric acid from the 
mixtures given above was readily obtained, but during the prelimi- 
nary stages of the work great difficulty was experienced in recovering 
the acid thus evolved, since its absorption in water can only be com- 
pletely brought about by rather elaborate systems of sprays, baffles, 
or diving walls, and even then it is impractical to collect all of the 
P 2 5 fumes in the form of strong phosphoric acid. These investiga- 
tors, therefore, decided to try out the Cottrell method of electrical 
precipitation i2 for collecting the volatilized acid, and after a good 
deal of preliminary experimentation built a small plant which proved 
conclusively that this process could not only be successfully employed* 
but possessed a number of advantages over the water-absorption 
system. This was the first time that the Cottrell precipitator was 
ever used for collecting a product purposely volatilized in order to 
apply this method of recovery. 
Since this same precipitator was used in the early experiments 
conducted by the senior author of this bulletin, a quotation descriptive 
of the apparatus and its auxiliary equipment taken from the article 
of Ross, Carothers, and Merz 43 is given below. 
COTTRELL ELECTRICAL PRECIPITATOR. 
The power used in the precipitator was provided by a 110-volt, 60-cycle, alternating- 
current motor-jrenerator set, and was stepped up by an 8 kilovolt ampere transformer 
capable of yielding voltages of 10,000, 20,000, and 40,000 volts. In these experiments 
the highest voltage only was used. From the transformer the current was changed 
to intermittent direct current by a small mechanical rectifier attached to the shaft 
of the generator and so adjusted as to operate in synchronism with it. The positive 
lead from the direct current side of the rectifier was earthed while the other terminal 
was connected to the wires suspended in multiple, one in each of the pipes used in 
the precipitator. Each pipe was then earthed. 
*y In the preliminary experiments made with this precipitator, sheet-iron pipes were 
used, 6 inches in diameter and 10 feet high. The precipitated acid, however, had 
such a corrosive action on the pipes that they had to be abandoned. Pipes were 
then constructed from ordinary 6-inch glazed terra-cotta tile, five sections being 
taken for each pipe. The precipitated acid gave to the inside of the pipes an effective 
conductive surface and in grounding the pipes it was found sufficient simply to bring 
the ground wire to the inside surface of each pipe at one point. Arranged in this 
way the terra-cotta pipes could be used just as efficiently in the precipitation of the 
acid as metal pipes and they had the very important advantage of being unaffected 
by phosphoric acid. Wires of monel metal were found to be least acted upon when 
suspended in the pipes, but nichrome wire also served quite well. Weights were 
suspended at the ends of the wires and individual oscillations were prevented by \a 
wire connecting all the weights. * * * 
42 Lodge, Sir Oliver, The Electric Disposition of Dust, Smoke, etc., Jour. Soc. Chem. Ind. 5, p. 57 
(1886); Cottrell, F. G., Electrical Precipitation of Suspended Particles, Jour, of Ind. and Eng. Chem. 3, 
fi. 542 (1911); Strong, W. W., Electrical Precipitation of Suspended Matter in Gases, Jour. Franklin 
nstitute 174, p. 239, Sept. 1912; Cottrell, F. G., Electrical Fume Precipitation, Trans. Amer. Tnst. Min. 
Eners. 43, p. 512 (1912); Cottrell, F. G. Problems in Smoke Fume and Dust Abatement, Smithsonian* 
Institution, Report, 1913, p. 653. Publication No. 1307 (1914): Howard, W. H., Fume Precipitation at 
Garfield, Bui Am. Inst. Min. Engrs. 49, p. 540 (1914); Nesbit, A. F., Theoretical and Experimental 
Considerations of Electrical Precipitation, Proc. Am. Inst. Elect. Engrs. 34, p. 507 (1915); Strong, W. W., 
Theory of Electrical Precipitation, Proc. Am. Inst. Elect. Engrs. 34, p. 220 (1915); Bradley, Linn, Prac- 
tical Application of Electrical Precipitation, Proc. Am. Inst. Elect. Engrs. 34, p. 523 (1915); Shmidt, 
W. A ., Cottrell Processes of Electrical Precipitation, Trans. Canadian Mining Inst., p. 110 (1915); Strong, 
W. W., Some Theoretical Aspects of Electrical Fume Precipitation, Trans. Am. Electro. Chem. Soc. 31, 
p. 415 (1917); Heimrod and Egbert, The Cottrell Processes in the Sulphuric Acid Industry, Chem Met. 
Eng. 19, p. 309 (1918): Gellert. N. H.. Electrical Cleaning of Blast Furnace Gas, Blast Furnace and Steel 
Plant 7, p. 334 (1919); Landolt and Pier, Air Cleaning bv the Cottrell Electrical Precipitation Processes, 
Bui. Am. Soc. of Heating and Ventil. Engrs., January (1920); Hesson, Landolt, and Heimrod, Pecent 
Applications of the Cottrell Processes, Eng. and Min. Jour. 112, p. 446 (1921). 
"Loc. cit. 
