The arc process, while a very interesting development from an electrochemical stand point and as the pioneer in the field, cannot cope with the cyanamide process either in cost of operation or in efficiency. One of the simplest forms of apparatus and at the same time one of the most satisfactory is that of Birkeland and Eyde. If an arc is struck be tween two high-tension electrodes, it imme diately tends to break down to a low-voltage arc, at a high current, but if the arc is placed between two powerful electromagnets the elec tromagnetic force will bend the arc out from the line of the electrodes, in a semi-circular form. As the arc spreads and becomes longer, the current drops and the voltage at the elec trodes increases and soon reaches a point where a second arc strikes across between the electrodes in the same manner, to be imme diately followed by others until the first arc formed reaches such a length that the voltage is no longer able to sustain it. Since an alter nating current is used, the succeeding arcs form on opposite sides of the electrodes and, with proper regulation, maintain a circular sheet of flame, composed of a series of arcs progress ing outward till they are extinguished either by becoming too long to be maintained by the volt age available or by the reversal of the electro motive force at the end of every half period of the alternating current.
This disc of flame, which is about 1.6 metres in diameter, is enclosed in a furnace and a current of air is forced out radially on each side of the arc. Furnaces are in operation, taking 3,200 to 4,000 kilowatts at about 5,000 volts, of which 3,300 to 3,900 volts are across the arc, the remainder being the drop in the series inductances. The frequency is 50 cycles, the power factor is 66 to 68 per cent and the current 940 amperes. The gas from the fur naces carries 1 to 1.2 per cent NO, and the yield of HNC., is about 67 grams per kilowatt hour.
The Paulding process is similar in princ ple to the Birkeland-Eyde, but differs in applica tion. Here the succession of arcs is produced not by magnetic deflection but by blowing the blast of air that is to be treated between the two electrodes, thus accomplishing the same end as the magnet of Birkeland and Eyde, except that the sheet of flame extends on one side of the electrodes only. The gas from the Paulding
furnace is somewhat richer than that from the Birkeland-Eyde furnace, but the yields are some what lower. Each furnace contains two 200 kilowatt arcs in series, at 4,000 volts, taking 140 amperes at a power factor of 70 per cent.
In comparison with these processes, the Schanherr process is of interest, this being based on a different principle. Both the Birke land-Eyde and the Paulding processes recognize the difficulty of maintaining a high-tension arc, particularly when subjected to a current of air, and so means are provided for securing a rapid succession of arcs as fast as they are extin guished. As a matter of fact, the arcs are made to overlap, so several exist in parallel at the same time. Schonherr, however, attacked the problem of maintaining a stable high-ten sion, high-current arc, working on the pnn ciple that if the air current were so introduced that it did not deform the arc, the discharge could be maintained in a stable condition. Paulding used the air current, at right angles to the arc, to draw the arc out to a greater length. Schonherr secured the same result without destroying the arc by introducing the air tangentially from all sides simultaneously so that it traveled with a helical motion in the direction of the arc. This neither deformed the arc nor overcooled it, allowing it to burn quietly. Working on this pr.nciple, the com mercial furnaces designed by Hessberger take 700 to 750 kilowatts, 3,500 volts and 290 amperes with a power factor of 66 per cent. The arc carried is from 5 to 7 metres (16 to 22 feet) in length. The gases carry 1.5 to 2 per cent NO and the yield is 68 grams HNC/ per kilowatt hour.
Ozone is a polymerized form of oxygen which may be produced by a silent electrie dis charge from a static electric machine, induction coil or very high voltage transformer, through oxygen or air.
It has powerful oxidizing and bactericidal properties and is extensively used in water and air purification.
Other Many other com pounds, too numerous to describe, are made by electrochemical processes of one kind or an other. For details of the principles involved in the operations of such processes. see ELECTRO