In 1899 Albert Nodon of Paris began experimenting with an electric rectifier which is now on a commercial footing. It is known as the Nodon electric valve, and it is claimed that it will give an efficiency of 75 to 8o% when used to transform single or polyphase currents into continuous currents . In the form used for transforming single- phase currents the valve is made up of 4 cells, each consisting of an iron cylinder with an insulating plug at the bottom through which is passed a cylinder formed of an alloy of zinc and aluminium . This cylinder is concentric with the iron tube and provided with a terminal at the lower end . The cell is filled with a saturated solution of ammonium phosphate, and a non-conducting shielding tube can be slid over the aluminium electrode to alter the exposed area . The valve is shown in section in fig . 12, and the 4 cells are arranged in a Wheatstone's Bridge fashion, as shown in fig . 13 . A and A' are the terminals to which the alternating current is supplied, C and Ct the terminals from which the continuous current is drawn off . The electrolytic actions which take place in the cells are as follows: When the alternating current passes in the positive direction from the zinc-aluminium cylinder to the iron cylinder there is formed instantly on the former a film of aluminium hydroxide; this film, presenting an enormous resistance, opposes the passage of the current . On the other hand, if the current passes in the opposite direction the film is reduced instantly and the current now flows . When used with polyphase currents the valve comprises as many times two cells as there are wires in the distribution . The cells must stand a pressure varying from 5o to 140 volts, and for higher pressures two or more valves in series are employed . The aluminium-iron electrolytic rectifier is not suitable for the rectification of very high frequency currents, because the chemical actions on which it depends involve a time element. vacuum or It was, however, discovered by J . A . Fleming that an vapour oscillation valve could be constructed for rectifying Rectifiers. electrical oscillations, as follows (Proc . Roy . Soc . Lond., 1905, 74, p . 476) In a glass bulb similar to that of an incandescent lamp a carbon filament is fixed . Around the carbon filament, but not touching it, is placed a cylinder of nickel connected to an external terminal by means of platinum wire sealed through the glass . If the carbon filament is made incandescent by an insulated battery (and for this purpose it is convenient to have the filament adjusted to be fully incandescent at a pressure ? of about 12 volts), then the space between the incandescent filament and the embracing cylinder possesses a unilateral conductivity such that negative electricity can pass from the incandescent filament to the cylinder but not in the opposite direction . Hence if the negative terminal of the filament and the terminal attached to the cylinder are connected to an oscillation transformer (see INDUCTION COIL) which supplies a high frequency alternating oscillatory current, the flow of electricity in one direction is cut out and the oscillatory current is therefore converted into a continuous current . Such valves have been employed by Fleming in connexion with wireless telegraphy . Wehnelt discovered that if a platinum wire was covered with oxide of barium or any of the oxides of rare earth metals, it possessed in the same manner, when used in a valve of the above type, an even greater power than incandescent carbon.
The explanation of this action is to be sought for in the fact that incandescent carbon in a vacuum or incandescent earthy oxides copiously emit negative electrons . A rectifier dependent upon the peculiar qualities of mercury vapour has been devised by Cooper-Hewitt for the transformation of polyphase currents into continuous currents . The three-phase transformer is made as follows: A large glass bulb (see fig . 14) has four iron electrodes sealed through the walls as positive electrodes and a negative electrode consisting of a pool of mercury in the bottom of the bulb connected with platinum wires sealed through the glass; the bulb is highly exhausted and contains only mercury vapour . The three iron electrodes are connected to the terminals of a star-connected polyphase transformer and one of them to the positive pole of a continuous current starting current, the connexions being shown as in fig . 15 . The mercury vapour is a non-conductor for low voltages, but if a sufficiently high voltage is placed on the mercury bulb by means of the continuous current it begins to conduct and if the three-phase current is then switched on the mercury vapour will allow the components of the three-phase current to pass when the mercury electrode is negative, not when it is positive. Hence for alternate cur-rent wave of the three-phase, supply is cut down and a continuous current can be drawn by the connexions as shown in fig . 15 for the purposes of supplying secondary batteries, arc lamps, &c . Owing to the fact that the mercury vapour ceases to conduct when the electromotive force on it falls below a certain critical value the valve will not work with single-phase currents but will work with polyphase currents at all voltage from too to woo or more and can transform as much as too amperes . It is stated to have an efficiency of 88 to 89 % . (See The Electrician, 1903, 50, p . 510.) A i1 A' (From the Electrical Times, by permission.) (From the Electrical Times, by permission.) Nodon Valve. the cells . Nodon Valve . Cooper-Hewitt Rectifier . A mechanical polyphase rectifier or rotary devised by Bragstad and La Cour is described in Der Kaskadenumformer, by E . Arnold and J . L . La Cour Stuttgart, 1904 . It consists of a three-phase induction motor coupled direct to a continuous current dynamo, the armatures of the two machines being electrically connected so that the three-phase current created in the rotor of the induction motor enters the continuous current armature and creates around it a rotary field . The connexions are such that the rotating field turns in a direction opposite to that in which the armature is turning, so that the field is stationary in space . From the continuous current armature can therefore be drawn off a continuous current and the device acts as a transformer of three-phase alternating current to a continuous current . The ordinary induction coil (q.v.) may be regarded as the trans-former for converting continuous current at low voltage into high voltage intermittent continuous current, but the difficulties of interrupting the primary current render it impossible to transform in this way more than a small amount of power . Where, however, high voltages are required, high potential transformers are used which are now built for the purpose of wireless telegraphy and the transformation of power to give secondary voltages up to 20,000, 30,000 or 6o,00o volts . Transformers have even been built to give secondary voltages of half a million volts capable of giving a 14 in. spark in air . These machines, however, must be regarded as more physical laboratory instruments than appliances for technical work . For description of one such extra high potential trans-former see H . B . Smith, on " Experiments on Transformers for Very High Potentials," The Electrician (1904), 54, p . 358 . A trans-former of this kind must invariably be an oil insulated transformer, as under extremely high voltage the air itself becomes a conductor and no solid insulator that can be put upon the wires is strong enough to stand the electric strain . (J . A ).