SMOKE AND DUST ABATEMENT COTTRELL. 667 



it will be attracted to the plate of opposite charge and will move the 

 faster the higher its charge and the greater the potential gradient 

 between the point and plate. 



Even if there are no visible suspended particles the gas molecules 

 themselves undergo this same process, as is evidenced by a strong 

 wind from the point to the plate, even in perfectly transparent gases. 

 The old familiar experiment of blowing out a candle flame by pre- 

 senting it to such a charged point is simply another illustration of 

 the same phenomena. 



As above indicated, the first step toward practicability was of 

 necessity a commercially feasible source of high-tension direct cur- 

 rent. The obstacles to building ordinary direct-current generators 

 for high voltages lie chiefly in difficulties of insulation, and if this 

 is avoided as to individual machines by working a large number 

 in series the multiplication of adjustments and moving parts intrudes 

 itself. On the other hand, high potential alternating current tech- 

 nique has in late years been worked out most thoroughly, and com- 

 mercial apparatus up to 100,000 volts and over has been available for 

 some years in the market. 



The procedure actually used in the installations described below 

 is diagrammatically illustrated in the early Patent Office drawing 

 (fig. 2), and consists in transforming the alternating current from 

 an ordinary lighting or power circuit P up to some 20,000 to 75.000 

 volts through the transformer (9, and then commutating this high 

 potential current into an intermittent-direct current by means of a 

 special rotating contact maker J driven bj^ a synchronous motor L. 

 This unidirectional current is applied to a system of electrodes in the 

 flues carrying the gases to be treated. In the particular form shown 

 in the drawing the wall of the chamber A itself acts as one electrode, 

 the other electrode C being suspended within it. The heating circuit 

 h and inlet for clean gas G are merely for protection of the insulation 

 from condensation of acid and moisture. 



The electrodes are of two types corresponding to the plate and point 

 in the experiment above cited. The construction of electrodes corre- 

 sponding to the plate presents no special problem, as any smooth 

 conducting surface will answer the purpose. With the pointed or 

 discharge electrodes it is quite otherwise, and the working out of 

 practical forms for these proved the key to the first commerciallj'^ 

 successful installations. 



In laboratory experiments when the discharge from a single point 

 or a few such was being studied fine sewing needles or even wire 

 bristles answered very well, but when it was attempted to greatly 

 multiply such discharge points in order to uniformly treat a large 

 mass of rapidly moving gas at moderate temperatures great dif- 



