A LECTURE DEMONSTRATION OF THE COTTRELL PRECIPITATOR HOSMER w. STONE, UNIVERSITY O F CALIFORNIA AT LOSANGELES The demonstration of the Cottrell Precipitator has been difficult for the average instructor because it required high voltage direct current. Recently, Norman McGrane of the Western Precipitation Company showed the writer that this demonstration could be made with the equipment usually available in chemical laboratories. Diagram I is practically self-explanatory. The 2" X 18" iron pipe is connected to one terminal of the secondary of an induction coil. The other terminal of the secondary leads to a wire or rod about 2 mm. in diameter extending down though the middle of the pipe. The end of the wire has been fused to asmall ball to avoid the point which would favor the formation of a spark a t this point. A six-volt battery supplies the current and is connected in series with an adjustable resistance to the primary of the induction coil. When the apparatus is properly assembled with the minimum resistance and the circuit closed there should be a spark between the center wire and the wall of the pipe. Any coil capable of producing this discharge is satisfactory. The resistance should then be increased until the sparking is just stopped while the coil point continues to vibrate. Industrial, installations are' run a t about eighty per cent of the discharge voltage. The connections arc made in such a way that the electrons pass from the wire to the pipe. Unless this be the case the smoke is not stopped. Hence, if it does not work one way the poles are reversed to produce the desired results. The smoke is produced by allowing compressed air to flow first over concentrated hydrochloric acid and then over concentrated ammonyum hydroxide. The white fumes of ammonium chloride, blown into the b o t t ~ m of the pipe, issue from the top in a great cloud. With a steady stream of the chloride entering a t the bottom its emergence at the top can be controlled a t will by completing and breaking the primary electrical circuit. It is desirable to have the tube in the ammonium hydroxide bottle as large as is convenient to prevent clogging. A little experimentation is usually needed to adjust the air pressure to the needs of a particular set up. The intensity of the smoke cloud may be increased by bubbling the air through the liquids if that is desired. The writer had difficulty with the points of the coil sticking when it was subjected to prolonged service. This was overcome by replacing the points with larger surfaced contacts from a Ford car. After the precipitator has been in operation for a time solid ammonium chloride may be shaken from the pipe by tapping it. This is an interesting
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part of the demonstration since the removal of the precipitated material is an important and often troublesome part of the industrial procedure. Cement plants with these installations are known to precipitate over one hundred tons of dust per day from the escaping gases. Diagram I1 indicates a modified demonstration in which the precipitation is made visible by the use of a glass tube. This is quite striking and in the dark the corona may be seen by those close a t hand. The glass tube, used in place of the iron pipe, is made a conductor by moistening the inside with concentrated sulfuric acid. The wire leading to the tube is extended to the inside wall to make contact with the film of aulfuric aeid. Fumes from hot concentrated sulfuric acid are blown up the tube and can be completely stopped by completing the circuit of the precipitator. Though this demonstration lacks the visible precipitate obtained from the iron tube and the ammonium chloride the clear vision and the possibility of seeing the corona make it a desirable adjunct to the other.
Since compressed air is not a t hand in many classrooms the writer tried other sources of smoke. A tank of carbon dioxide was used in place of the compressed air and found to give an excellent smoke. However, when once the current had been turned on and the smoke stopped, the smoke would not again start up the tube when the current was turned off but would flow out at the botton. This was doubtless due to the greater density of the carbon dioxide, for sulfur dioxide behaved in the same way. Fanning and placing a gas flame a t the bottom af the tube helped to keep the fumes rising in the tube but these gases were a t no time as satisfactory as the compressed air. Doubtless a tank of ammonia would work very well, though this was not tried. Smoke from burning turpentine worked very well as far as producing the smoke was concerned but the apparatus would not stop this smoke.
Many readers will doubtless object to the apparent claim that a direct high voltage current is obtained from an induction coil. It is a wellknown fact that, though current from such a coil is of alternating character, there is a partial rectification. Probably there is a resultant in one direction which accounts for the success of the demonstration. Aside from the fact that this demonstration arouses much interest and illustrates a scientific development of great industrial importance it gives the student visual evidence of a chemical principle usually rather difficult to demonstrate. Ordinarily the student has to take it on faith that the dispersed particles of a colloid actually carry a charge. The experiment with the Cottrell Precipitator demonstrates rather conclusively that the dispersed particles of the ammonium chloride smoke are charged.
