A ROTATING CATHODE BY C. W. BENNETT
In order t o obtain samples of metals and alloys, by precipitation on a rotating cathode, with high current densities, for the purpose of studying their physical properties, a cathode has been designed and built, which will perform six thousand revolutions per minute, carry three hundred amperes, and operate continuously under these conditions. A sketch of the cathode, mounted with motor, rheostat, and accessories, is shown in Fig. I . The whole apparatus was mounted
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on a frame of heavy boards, 2” X 8” well braced, and fastened, to prevent vibration. An inch and a half steel rod was held by set screws in the two cast steel holders which were screwed securely t o the vertical face of the support, Fitting over this steel rod were two collars, held in place by heavy set screws a t the back. The extensions on these collars were notched
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to receive the two steel bars H, H, which were 24” x 4” x ’/,”. In front of these, and fastened t o the collars were heavy steel plates, through which cap screws passed, clamping the bars H, H, at any desired position in the horizontal plane. I n order t o prevent vibration and change of adjustment, two heavy bars or stiffeners were bolted vertically between
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the ends of the bars H, H. By means of this arrangement, i t may be seen that adjustment could be obtained around in the horizontal plane, and upward and downward in the vertical plane. The cathode F consisted of a length of aluminum pipe, one inch outside and three-quarters inch inside diameter and nine or ten inches long (furnished by Mr. Blough, of the Aluminum Company of America). A rubber stopper served t o close the bottom. This tube was held in
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an inch hole in the brass connector D, by a set screw. The top end of the connector was tapped to receive a 5//,” shaft. The cathode and connector are shown in detail in Fig. 2 . The shaft and cathode were mounted on the steel bars H, H, by two bearings, a simple babbited one at the top, with a ball bearing A, and a roller bearing B, a t the bottom. The shell of the roller bearing B, fitted into a water-cooled collar C, which being bolted to H held the bearing in place. At high speed of rotation it was found necessary t o cool this bearing, hence the hollow collar was cast. A detail of the bearings, water-cooled collar, and an assembly of these are shown in Figs. 3 and 4. In using generator circuits where the positive
side is grounded for lightning protection, it is necessary to insulate the pipe delivering water to this collar. This was accomplished by inserting in the line a short length of garden hose. The discharge pipe was prevented from touching the sewer pipe by slipping a rubber tube over the end for a short distance. Above the ball bearing A, Fig. I , a collar was fastened to the shaft. This held the shaft up in the bearings.
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This collar ended a t the top in a horizontal plate which was screwed to the cone pulley, serving t o hold the latter in place. The electrical connection was made by means of a copper bar 3-tenths of a square inch cross section, bolted to H, just behind the collar C. The contact surface was slightly over three square inches. On opposite sides of D, and receiving the current from the cathode, were brushes E, consisting of thin plates of copper laid one upon the other, aggregating
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sufficient size to carry 300 amperes. These were mounted, as seen, in holders which were in turn bolted t o the front of H. The anodes G, G consisted of two bars of metal about 7” long, 2‘/,N wide, and I ” thick. To these were bolted strips of copper which extended through a narrow slit in the cover of the container for the electrolyte. These strips were connected by another strip to which the electrical connection was made. The cathode F rotated in a:’,II hole in the center of the cover. The cover consisted of a circular board I ”
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thick, with a circular strip screwed to it, which fitted snugly inside the two-gallon earthenware jar which was used for container. This arrangement gave practically a seal for the jar so that there was no loss of electrolyte. Then too the anodes acted as baffle plates to break the excessive rotation of the liquid, the most rapid rotations being made without splashing the solution out. The motor for rotating the electrode was mounted 011 the same frame with the apparatus previously described. It was found to require about one horse power t o rotate the cathode, 6000 R. P. M. when immersed to a depth of about five inches in the electrolyte. Where a rheostat was necessary, the arrangement K, Fig. I , was found to be handy, flexible, cheap, and very compact. A post of wood held a spiral of No. 15 nickel wire by means of small screw eyes spaced equally around it. The wire was thus held out from the wood so that water could run in at the bottom, circulate around the wire and discharge a t the top. From various points along the nickel spiral, copper leads were brought out to the binding posts at I,. By changing these connections which could be done by a sliding switch the current could be raised to a desired point gradually. This device carried 150 amperes and dissipated about I O kilowatts of energy a s heat, without the slightest inconvenience. The wire would, no doubt, carry 300 amperes. However, for currents of 300 amperes a separately excited machine operating a t any desired voltage was used and hence the resistance was cut out. The total volume of this rheostat to carry 300 amperes need be only about 1/4 of a cubic foot. The aluminum as shown in Fig. 2 was found to be an ideal metal upon which t o deposit other metals, it being covered with a voluntarily acquired film of oxide, which obviates the necessity of treating the cathode to prevent sticking of the deposit. The metal of course is deposited in a cylindrical shape of any desired thickness about the pipe
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of aluminum. In order t o get this cylinder of metal off whole, a thin shell of Rose’s metal was cast around the aluminum. When the deposition was complete, the whole was dipped in boiling water, the alloy melted, and the metal cylinder lifted off. However, for physical tests, it being unlikely that the whole tube would be desired, the metal was deposited directly on the aluminum, and the following method adopted, for getting strips of the metal for tensile strength and ductility tests. As can be seen from Fig. 2 , the connector holding the aluminum pipe was unscrewed from the shaft, the rubber stopper was removed from the lower end of the pipe, and a brass plug, which was centered ” and tapered from the “centered” end, was driven in. The taper was so arranged that by slight pressure the plug fitted tightly inside the aluminum pipe. This done, the connector was chucked” in a metal lathe, the center pin run up on the centered plug, and a section about an inch wide turned down in the center A, A, Fig. 2 , to constant thickness. The whole was then removed and placed, in the same way as before, in a milling machine. Cuts were then made lengthwise through the metal as shown in Fig. 2 section A A. These strips were then pried off with a screw driver, the edges dressed down with a file, when, after measurements of the cross section, they were ready for physical tests. It was found more economical both of time and aluminum pipe, to cut almost through the copper to the aluminum below, on all save the last cut, where the fine adjustment of the machine was used and the copper cut completely through. After removing the metal the strips were broken apart by bending back and forth upon themselves. If a deep cut be made in the aluminum pipe, it is necessary, in order to get a perfect cylinder deposited the next time, t o replace the pipe by a new one. In conclusion, it may be said that ( I ) A revolving cathode has been built which ( a ) will perform speeds up to 6000 R. P. M.,(b) will carry 300 amperes,
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giving a current density of about 3500 amperes per square foot when 4 inches of the cathode is receiving a deposit. (2) The metal can be removed in good shape for testing physical properties without subjecting t o strain of bending or heating. This cathode was built a t the suggestion of Professor Bancroft. Carttell U niversitj.
