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There has, however, been interest among railroad operators in returning to DC use at higher voltages than previously used. At the same voltage, DC often has less loss than AC, and for this reason high-voltage direct current is already used on some bulk power transmission lines. DC avoids the electromagnetic radiation inherent with AC, and on a railway this also reduces interference with signalling and communications and mitigates hypothetical EMF risks. DC also avoids the power factor problems of AC. Of particular interest to railroading is that DC can supply constant power with a single ungrounded wire. Constant power with AC requires three-phase transmission with at least two ungrounded wires. Another important consideration is that mains-frequency 3-phase AC must be carefully planned to avoid unbalanced phase loads. Parts of the system are supplied from different phases on the assumption that the total loads of the 3 phases will even out. At the phase break points between regions supplied from different phases, long insulated supply breaks are required to avoid them being shorted by rolling stock using more than one pantograph at a time. A few railroads have tried 3-phase but its substantial complexity has made single-phase standard practice despite the interruption in power flow that occurs twice every cycle. An experimental 6 kV DC railway was built in the Soviet Union.
Which type of power does require three-phase transmission?
A: AC

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In late 1203, John attempted to relieve Chateau Gaillard, which although besieged by Philip was guarding the eastern flank of Normandy. John attempted a synchronised operation involving land-based and water-borne forces, considered by most historians today to have been imaginative in conception, but overly complex for forces of the period to have carried out successfully. John's relief operation was blocked by Philip's forces, and John turned back to Brittany in an attempt to draw Philip away from eastern Normandy. John successfully devastated much of Brittany, but did not deflect Philip's main thrust into the east of Normandy. Opinions vary amongst historians as to the military skill shown by John during this campaign, with most recent historians arguing that his performance was passable, although not impressive.[nb 8] John's situation began to deteriorate rapidly. The eastern border region of Normandy had been extensively cultivated by Philip and his predecessors for several years, whilst Angevin authority in the south had been undermined by Richard's giving away of various key castles some years before. His use of routier mercenaries in the central regions had rapidly eaten away his remaining support in this area too, which set the stage for a sudden collapse of Angevin power.[nb 9] John retreated back across the Channel in December, sending orders for the establishment of a fresh defensive line to the west of Chateau Gaillard. In March 1204, Gaillard fell. John's mother Eleanor died the following month. This was not just a personal blow for John, but threatened to unravel the widespread Angevin alliances across the far south of France. Philip moved south around the new defensive line and struck upwards at the heart of the Duchy, now facing little resistance. By August, Philip had taken Normandy and advanced south to occupy Anjou and Poitou as well. John's only remaining possession on the Continent was now the Duchy of Aquitaine.
What did John attempt to relieve in late 1203?
A: Chateau Gaillard

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Florey and Chain succeeded in purifying the first penicillin, penicillin G, in 1942, but it did not become widely available outside the Allied military before 1945. Later, Norman Heatley developed the back extraction technique for efficiently purifying penicillin in bulk. The chemical structure of penicillin was determined by Dorothy Crowfoot Hodgkin in 1945. Purified penicillin displayed potent antibacterial activity against a wide range of bacteria and had low toxicity in humans. Furthermore, its activity was not inhibited by biological constituents such as pus, unlike the synthetic sulfonamides. The discovery of such a powerful antibiotic was unprecedented, and the development of penicillin led to renewed interest in the search for antibiotic compounds with similar efficacy and safety. For their successful development of penicillin, which Fleming had accidentally discovered but could not develop himself, as a therapeutic drug, Ernst Chain and Howard Florey shared the 1945 Nobel Prize in Medicine with Fleming. Florey credited Dubos with pioneering the approach of deliberately and systematically searching for antibacterial compounds, which had led to the discovery of gramicidin and had revived Florey's research in penicillin.
Who came up with a way to quickly produce penicillin?
A: Norman Heatley

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Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards since the holes must pass through all layers to the opposite side. Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB.
What type of boards end up with limited routing space in through-hole manufacture?
A:
multi-layer