A SIMPLE A.C. RECTIFIER.

Radio Record, Volume I, Issue 6, 26 August 1927, Page 14

A SIMPLE A.C. RECTIFIER.

Correspondents are makiug inquiries regarding chemical rectifiers for 280 volts alternating current, so the following brief description will be given. A glass jam jar about 2} by 4} inches makes a suitable container, and all that is required in addition is a wooden cover of some kind to keep out dust and to hold the plates, which must be kept as far apart as possible in order to ensure proper rectification. The two plates are of lead and aluminium respectively, and the dimensions of these should be about 4tin. by jin. A hole is drilled in one end of each and a screw through this will affix each to the sides of the cover, the connecting wires being either clipped round the screw under the plate or soldered to each (special solder for the aluminium). The lead must be well cleaned to present a bright surface. The aluminium is the vital part of the rectifier, and unless this is’ pure metal, cannot perform its duty satisfactorily. ‘hin sheet aluminium of pertect purity can be obtained, and

will last quite a long time if the precaution is taken of cutting a strip about an inch pide and folding tt round the aluminium plate so that the level of the liquid comes at about the centre of this reinforcement. This is done because most of the corrosion of the plate takes place at the surface of the liquid. Put into the jar sufficient tap-water to reach about liins. from the top. Ammonium phosphate is then put in and allowed time to dissolve, or stirred to quicken the process. A saturated solution is required, so that a small quantity of the chemical should remain undissolved at the bottom of the container. Occasionally, as the water has evaporated, more woter should be added to keep it at the proper level. If the aluminium is pure it will only show a luminous glow in the dark when rectifying properly; if violent sparks are dancing all over the surface, rectification is not as perfect as it should be. No trouble will be experienced with the lead plate, but before the rectifier will work properly it must be run for an hour or two so that the plates will "form," and this should be done as a precaution for a minute or so before each charging operation, if more than a day or two has elapsed since the last time. of using, because the formed oxide is

not permanent, When the forming is being carried out the battery is not connected up, but the aluminium side of the rectifier and the main not connected with the lamp resistance are connected together’ with or without a suitable re‘sistance in place of the battery. In order ‘to cut down the voltage where such is 2% advantage, it is a good plan to have two rectifier cells in series, the lead of. one being connected to the. aluminium of the other. This is also a safeguard, as if one cell happens not to be rectifying, the other may continue to do so and save any trouble. When charging the accumulator B battery recently described in this column, a 60-watt lamp should be included in series with the rectifier, and for smaller accumulators a lamp of less power may be used, whilst for voltages of 46 or 56 it may be found an improvement to include in the circuit next of No. 36 michrome resistance wire space wound upon a piece of red fibre or other noninflammable material. Such a resistance only gets slightly warmed up, but

it should be put upon the material men--tioned. Should this be found necessary the fibre can be cut slightly narrower than the baseboard and fastened underneath out of the way, a strip of wood being screwed under each end of the base to raise it up sufficiently high. The diagram shows a two-way switch that can be put on contact for a minute to start rectification and then put over to 2 to charge the battery. The aluminium is always the positive electrode, and when charging is connected to the positive side of the battery, , When using any apparatus such as this connected to the electric mains it is a good plan to go to a small amount of extra trouble to include a fuse on both mains. A piece of fibre about two inches square is drilled with six 1-8th inch holes’ as shown. The two centre holes are for. screwing down to a strip of wood. The other holes are fitted with 1-8-inch. brass bolts with a washer and nut on top, to which connections are made. A short piece of 40's copper wire (thinner if handy) is carefully cleaned and a piece clipped under each pair of washers, Atl wires in the circuit should be insulated where possible. If the switch is provided it can also be fitted with an "off" position for convenience.