Fig. 1,135.—Elementary diagram of mercury arc rectifier connections. A, A´, graphite anodes; B, mercury cathode; C, small starting electrode; D, battery connection; E, and F reactance coils; G and H, transformer terminals; J, battery.
Ques. Describe the construction and operation of a mercury arc rectifier.
Ans. Fig. 1,135 is an elementary diagram of connections. The rectifier tube in an exhausted glass vessel in which are two graphite anodes A, A´, and one mercury cathode B. The small starting electrode C is connected to one side of the alternating circuit, through resistance; and by rocking the tube a slight arc is formed, which starts the operation of the rectifier tube. At the instant the terminal H of the supply transformer is positive, the anode A is then positive, and the arc is free to flow between A and B. Following the direction of the arrow still further, the current passes through the battery J, through one-half of the main reactance coil E, and back to the negative terminal G of the transformer. When the impressed voltage falls below a value sufficient to maintain the arc against the reverse voltage of the arc and load, the reactance E, which heretofore has been charging, now discharges, the discharge current being in the same direction as formerly. This serves to maintain the arc in the rectifier tube until the voltage of the supply has passed through zero, reversed, and built up such a value as to cause the anode A to have a sufficiently positive value to start the arc between it and the cathode B. The discharge circuit of the reactance coil E is now through the arc A'B instead of through its former circuit. Consequently the arc A'B is now supplied with current, partly from the transformer, and partly from the reactance coil E. The new circuit from the transformer is indicated by the arrows enclosed in circles.
Ques. How is a mercury arc rectifier started?
Ans. A rectifier outfit with its starting devices, etc., is shown in figs. 1,132 to 1,134. To start the rectifier, close in order named line switch and circuit breaker; hold the starting switch in opposite position from normal; rock the tube gently by rectifier shaker. When the tube starts, as shown by greenish blue light, release starting switch and see that it goes back to normal position. Adjust the charging current by means of fine regulation switch on the left; or, if not sufficient, by one button of coarse regulation switch on the right. The regulating switch may have to be adjusted occasionally during charge, if it be desired to maintain the charging current approximately constant.
Capacity.—The unit of capacity of a storage cell is the ampere hour, that is, the ability to discharge one ampere continuously for one hour. For instance, a 100 ampere hour battery will give a continuous discharge of 12½ amperes for eight hours. It should theoretically give a discharge of 25 amperes continuously for four hours, or 50 amperes for two hours, but in reality, the ampere hour capacity decreases with an increase of discharge rate.
It requires, theoretically .135 ounces of metallic lead on either element reduced to sponge lead or to lead peroxide to produce one ampere hour; in practice, from four to six times this amount is required.
The reason for this is because it is impossible to reduce all the active material, to bring every particle in contact with the electrolyte, or to cause every part to be penetrated by the current.
Experiments show that from .5 to .8 ounces of sponge lead, and from .53 to .86 ounces of metallic lead converted into peroxide, are required on their respective elements to produce a discharge of one ampere hour at ordinary commercial rates.