Mainly about Construction

Radio Record, Volume I, Issue 12, 7 October 1927, Page 14

Mainly about Construction

BY

MEGOHM

CHARGING THE 1122-VOLT B ACCUMU-: . LATOR , : _ HOW IT CAN BE DONE AT 20 VOLTS BY MEANS OF SMALL ADAPTER

(Continued from Last Issue.) | This device, the description of which was commenced in last week’s "Record," is an attachment to the 112volt B accumulator described in Nos. 2 and 8 of the "Récord." By meansof this idea a high-tension battery may be charged at low voltage by a full-wave valve rectifier intended for A battery charging, and giving about 20 volts. For the convenience of readers who have not seen the battery article, this description will be made as complete as possible, so that it could be constructed for attachment to any existing B battery. THE DISTRIBUTION PANEL. T'igure 1 shows the ebonite distribution ‘panel 64 by 3} inches, which is already a part of the B accumulator. The extra attachment is the strip of ebonite at the top, through which are inserted small copper mercury cups, into which suitable connectors are placed, so that the rows of cells in the battery are connected in "series" for discharging, aud _ in ‘parallel’? for charging. It «might here be mentioned that the use of mercury about a radio set is not to be recommended in a general way, but, used in the way here indicated. no trouble can possibly be caused, and the contact obtained is more certain and reliable than can be obtained by other means. The idea conld certainly be carried out with sockets and split pius for contacts, but the accurate drilling required to do this would prove a stumbling-block to most amateurs. A panel arranged as shown is a convenient attacliment for any B battery when experimental work is carried eut, and varying voltages are required, as the plug-in system is so much more convenient than loose clips. Any number of additional voltages can be orovided by inserting extra sockets connected to a wire tapping the row of tubes or cells at the required point. It will be noticed that four flexible leads come through holes in the pancl, and to the end of each of these a split plug is attached, which may be fitted with a short piece of 38-8-inch bonite rod as a handle. The leftand flex is the negative contection, and forms a handy aeans of cutting off the H.T. : ‘ft any time without taking out the Ositive plugs, which, however, must lways be taken out when the battery being charged. When not in use, 4e plugs are inserted into holes drilled ¢ the strip ot wood below panel, as sown. The fuse provided on each dltage to protect valve filaments is a ise precaution. The fuses are made ¢ pasting a strip of cigarette tinfoil on a piece of writing paper, and then cutting this into the finest shreds vith sharp scissors. The best cuttings tbout an inch or three-quarters long tan be stuck with seccotine to a strip xf strong paper, trimmed to size, and the ends slipped under the washers provided, beld down by jin. brass bolts through the panel, with the uuts out- |

side, handy for replacement of fuses. The lower end of the fuses is connected to the corresponding terminal by a Strip of thin brass. All connections are shown in the wiring diagram published last week. THE MERCURY CUPS We now come to the portion that is the actual addition to the battery as already specified. A full-size temiplet of the eup panel is given, so that the ebonite can be marked out accurately by placing the paper over it and punch- —

ing each centre The two holes marked A are jin. diameter, for the purpose of taking the brass bolts that hold a strip of ebonite 5} by } inches, placed underneath to hold the cups in position. This strip is drilled with two holes in corresponding position. The hole at either end marked B is to take a screw to hold the ebonite down to the wooden side supports. ‘The most suitable article to serve for mercury cups is empty .22 rifle cartridge cases. Mercury can do no harm in contact with copper, so these

serve the purpose admirably and shouid be easily obtained. Twenty will be required, all being selected for being of correct shape and unbent. ‘These cases are now cleaned by placing them to soak in battery strength acid for an hour or so, then washed and rinsed well in water. Now, with a knife round the end of a piece of stick to fit into the cups, placing a pinch of pumice powder in each and working the stick round to clean the inside. The outside ‘must also be brightened by rubbing with

pumice and water ona rag. Small tags ot copper foil about fin. long are now cut the width of bottom of cups at one -end, tapering to almost point at the other. A cup is now soldered to the broad end of each tag, and a (22’s enamel wire to the other end. | Kight of these wires must be long enough to reach the front negative plate of each row of tubes, and the other wires long enough to connect to back of panel. The cartridge cases measure about five-eighths inch long and just under a quarter-inch wide, | toe .

nn oo oo and it will be found that a fifteen sixty-fourths twist drill makes a hole that is a splendid fit. It is not an easy matter to drill a row of holes so that they all line up exactly, unless special precautions are taken, and the best way to ensure accuracy is to use the drill to make a hole through a 38-16 piece of brass, Clamp this in turn over the place where a hole is required, and drilling through the hole in the brass, the drill is bound to pierce the cbonite in the proper place. When these and the our extra holes laye been drilled, assémbling may be done by placing the cups in the ecbonite held downwards, placing the holding strip on top and bolting it in place. If your battery is already built, the panel need only be unscrewed and the top tilted forward slightly. The cup panel cau tlen be held on edge, cups facing outwards, and the leads from positive cups connected to the back of tapping sockets, and other conuections made according to diagram. The hole for extra charging terminal should be drilled before panel is unscrewed, Now the cup panel can be folded back, and the negative wires passed through slot to tubes. The positive wires to sockets must be sufficiently. long to allow of front anel being lowered an inch, This incl space allows of the connecting wires inside being regulated not to touch each other after both panels are fastened in place, then the space is closed by a_ strip of wood, as shown. Rnamelled wire is the best to use, as the spray from an accumulator soon rots colton covering. In adjusting connecting wires they should be arranged to ‘fall’ away from the panel so that the moisture may not run down and corrode brass nuts. A slight U bend made in any wires prening from battery to panel will prevent any drops of moisture from running down to the latter. THE CONNECTORS. igure 2 shows the "series"? connector or "‘comb" as they may conveniently be called. ‘Iwo of these will be required, one for charging, the other for discharging. The combs cousist of a piece of close-grained wood 38-8-inch thick, 5 5-S long, and 1 1-8-inch wide. These are finished off with glass-paper and given a coat of shellac yarnish, and are then marked for position of holes with the paper templet, two rows of ten holes each, just as on the cup panel, but the holes must be drilled to take the 14’s copper wire which is used in making the connecting staples. ‘The diagrams below give the connections on the combs. There is a projecting wite below each hole, except those marked XX. The staples are bent with a pair of square-nosed pliers, and must project about ?-incli below wood. ‘The best way is to make them too long, afterwards trimming equal. In the series comb most of the staples run diagonally, as shown, with the exception of one at each end. In the parallel or charging comb, all negative cups down one side, and all positive on the other side, are to be connected together, and this may be done in any way that seems most convenient, The method shown is to screw a strip of thin brass along each side, then drill this from underneath, the holes in wood guiding drill. Black lines show tops of staples, but there is also an odd projection at one end of each brass strip. The staples are now pushed through, and the tops s« ed

to the brass strip ‘The two holes being missed at the ends, cuts out all connection to the set when chargin,z is being done, thus absolutely preventing current accidentally reaching the set from this source. GENERAL REMARKS. ‘the charger leads may remain permanently conuected to the charging terminals, and a double-pole, doublethrow switch can be provided as shown in diagram, to charge either A or B battery as required, being held in neutral position when not required. In case of the charger being accidentally switched over to the B battery with the series comb in position, nothing would heppen, as this comb cuts off all connection to the charging circuit. When the battery is not in use the series comb can be taken out, so that the highest voltage is then only that of one tow, 14 volts, which reduces zeny chance of leakage. As there are seven cells in each row, the total voltage of these will be 14 volts, so that: a charger giving about 20 volts will answer well, current being regulated by a short. piece of. resistance wire, if required. Half amp. or » little more, will be a good charging cirrent, and charging should be complete in about an hoy and a half, The cups are not to be more than

VAATOURUELATERUSEDDSE YTD EA EEEE TSA UCHR TRA half filled with mercury, and the Iess the better, provided that it is not much more than one-eighth inch deep. The wire connector takes up space, and if the cups are too full the mercury will spill over when, they are put in. ‘The best way to fill the cups is to take a cartridge case as used for the cups, cut it down to the depth the mercury is to be, twist a bit of 20’s wire round for a handle, aud use as a ladle to get just the right amount and place in each cup. A shilling’s worth of mercury is more than sufficient for the lot. To adapt this idea to other batteries, say of 24-volt units, the connector in the centre of each unit is cut, giving 12-volt units. The positive and negative end of each 12-volt group is then connected to its proper cup, which will «be numbered according to the voltage, 12, 24, 36, etc., and a charger giving more than 12 volts, say 16 or more, may then be used In case it is desired to vary the method of construction in any way, it should be mentioued that the only metal that cau safely be used in permanent contact with mercury is copper. It eats away brass or zinc, and solder it dissolves in a very short time, so that it cannot be safely used for many purposes for which it would otherwise be well suited where absolute contact is essential. ht 2D So = [6S Oe So a oe: ee ee a ee

. =? UE ge in quality. The best eftects come when one instrument has a greatly differing tone to the other. One will look after the rich deep effects, while the other will give you brilliance A horn model in tandem with a hornless type should give you this combination. The best balance reproduction comes with two loudspeakers of similar resistance, else one may be greedy and ovérpower the other. The two loudspeakers, one high and one low in tone, may be used separately tc get the best results from solo broadcast items The low-toned instrument will enrich a violin or piccolo, while the shriller loudspeaker will add brilliancy to a heavy bass voice. eg This method has been extensively used for some time in both Britain and America, and it undoubtedly gives the best reproduction obtainable under present circumstances, neither the horn nor the cone speaker being as good by itself, ; "‘Megohm" lias been testing out the idea for about three weeks ;with an "Ellipticon’ double-action cone speaker cud a medium-sized horn speaker, and finds it to be all that is claimed for it. One advantage is that neither speaker is overloaded, and yet both are giving maximum undistorted volume. The melody, emphasised by the horn, stands out agaitist the deep sonorous bass et RCS eee