Charging through a Mercury Arc Rectifier
The Mercury Arc Rectifier is a cheap and simple instrument for charging accumulators from a high tension, alternating current supply.
It may be described as an electric lamp in which vapour of Mercury, rendered highly incandescant by the passage of the electric current, is the source of light, this lamp having the singular property of converting, quite automatically, an alternating current into a direct current.
Its action is based on a principle analogue to that of the chemical rectifier, that Mercury vapour will permit current to flow in but one direction.
The tube in which the transformation takes place is an exhausted gas vessel shaped as shown below, and containing two anodes, A, (positive electrode), connected to the alternating side of the circuit.
One anode is at each side and well above the bottom of the tube; and a cathode, (negative electrode), forming the positive of the direct current side, is at the bottom of the tube, C, on which a small quantity of Mercury, D, rests. There is further a small starting anode, E, at the right, its function being to form an arc to start the rectifier.
This is done by shaking the tube slighty by means of the handle on the holder, (the Mercury at D and E thus becoming connected), and this auxillary arc only remains for just a few seconds, just long enough to start the main arc.
The Mercury on becoming vapourised, rises into the bulb, is condensed in relatively cold glass at the top and runs down the sides in the form of metallic Mercury.
The sustaining coil or inductance coil, F, is a most essential part of the apparatus.
It is connected directly across the the alternating current supply mains, A, and furnishes from this neutral point the negative side at, ??, of the direct current circuit, ?? shows the positive side of the direct current circuit and K, the accumulator.
The standard Mercury Arc Rectifier has a capacity of 30 Amps at 220 Volts. It is adapted for a pressure of 110, 220 or 330 Volts on the alternating side and the direct current can be varied from 18 to 315 Volts.
When connected to a 220 Volt supply the range of voltage on the DC side will be from 45 to 110 Volts.
Where a small number of cells are used, the pressure required may be between 18 and 45 Volts and to obtain this the 110 Volt alternating supply has been found most satisfactory.
One of the chief advantages of the Mercury Arc Rectifier is the possibility of using for all night charging on account of the absence of necessity for supervision, as any unusual drop in the circuit, or any back pressure from the battery ruptures the arc.
If an attempt were to be made to start below the voltage of the battery the arc would not form, so that no combination of circumstances or lack of knowledge on the part of the operator will cause damage to the battery.