Sources of Direct Current

Rectifiers without moving parts are aluminium or tantalum rectifiers, vacuum tube rectifiers, contact rectifiers and mercury-arc rectifiers. The first two types of rectifiers, the function of which has already been described in the theoretical section of this book are suitable for laboratory purposes only. Likewise vacuum tube rectifiers can also be used where a medium output is required these tubes act as valves allowing the alternating current to pass through in one direction only they are built for both low (up to 100 V) and high voltage (up to 10 000 V). 

For large electrochemical installations mercury-arc rectifiers are used whioh have proved valuable whereever strong direct current and relatively high voltage is required (above 300 V). In this case there is a loss of energy whioh is in proportion to the voltage necessary to overcome the arc resistance (20 to 30 V). As this voltage depends neither on the current nor on the total output 

G— Single phaeo mercury-arc rectifier, Ax A, — Anodes, K — Cathode, , t — Alternating current Input, +,-----Direct current output, D — Chok- 

Current fluctuations are damped by means of choking coil D connected in series. The rectifier starts operating by means of auxiliary electrode H which is connected to anode. 1, over the resistance W. When the rectifier is started it must be tilted so that the two pools of mercury accumulated at H and K merge into one. On restoring the rectifier to its normal position the pool of mercury is divided once again in two parts and gives rise to an electrical spark at the point of interruption the heat evolved by the spark causes that a certain amount of mercury, sufficient to ignite the arc, evaporates. 

Large rectifiers (Figs. 56 and 57) for the rectification of three-phase currents are enclosed in metal chamber Z with a flat dish containing mercury K placed on the bottom of it. 

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Two parallel plates of conducting material separated by an insulation material, called the dielectric, constitutes an electrical condenser. The two plates may be electrically charged by connecting them to a source of direct current potential. The amount of electrical energy that can be stored in this manner is called the capacitance of the condenser, and is a function of the voltage, area of the plates, thickness of the dielectric, and the characteristic property of the dielectric material called dielectric constant. 

Direct Current (DC). This current is transmitted for industrial uses only in exceptional situations. The most common sources of direct current are storage batteries and industrial devices called rectifiers, in which alternating current is changed (rectified) to direct current, as is used in electrolytic cells for the manufacture of chlorine gas, magnesium, aluminum, and a few other chemicals. The direct current is flowing from the source through the user application and back to the source, in one direction. The motor is primarily used for speed control of selected equipment. 

Forced Drainage form of drainage in which the connection between the protected structure and a traction system includes a source of direct current. 

The essential requirements for a constant-current electrolytic determination — a source of direct current (which may be a mains-operated unit producing a rectified smoothed output of 3-15 volts), a variable resistance, an ammeter (reading up to 10 amperes), a voltmeter (10-15 volts), and a pair of platinum electrodes — can be readily assembled in most laboratories, but if a number of determinations are to be performed a commercial electrolysis unit will doubtless be preferred. This will be equipped with rectifier, a motor drive for a paddle-type stirrer or with a magnetic stirrer, and a hotplate. 

A galvanic cell produces direct current, which flows in one direction. The mains supply at your home is a source of alternating current, which changes direction every fraction of a second. Explain why the external electrical supply for an electrolytic cell must be a source of direct current, rather than alternating current. 

The electrodes are connected to a suitable variable source of direct current (Note 6), the magnetic stirrer is started, and a potential of about 50 volts is applied. This results in a current flow of 1-2 amperes. The solution soon comes to boiling the voltage is then regulated so that a rapid reflux is maintained (Note 7). 

The scheme of experiments is shown in Fig. 31 (see Ref. [75]). The gel sample was put between two graphite electrodes. We used the source of direct current with current density 6.5 mA/cra2 which was automatically maintained at the preselected level. 

The electrodes are connected to a suitable source of direct current, 10 amperes of which is allowed to pass through the cell. To the solution there is now added 40 g. more of monoethyl sebacate in portions of 10 g. each over a period of ten minutes (Note 6). The temperature of the cell is held below 50° by running cold water through the cooling coil (Note 7). 

Because the reaction product (solid PbS04) adheres to the surface of the electrodes, a "run-down" lead storage battery can be recharged by using an external source of direct current to drive the cell reaction in the reverse, nonspontaneous direction. In an automobile, the battery is continuously recharged by a device called an alternator, which is driven by the engine. 

Immediately upon connecting the cell to a source of direct current, a deposit of gray metallic zinc appears on the surface of the cathode and bubbles of chlorine gas appear at the surface of the anode. A simple chemical test for chlorine may be made by leading this gas into an aqueous sodium iodide solution, whereupon the solution assumes a yellow color caused by displacement of iodine by chlorine. Accordingly, it is concluded that the products of the electrolysis of a zinc chloride solution are elemental zinc and elemental chlorine, and the next problem is that of explaining the mechanism by which these products may be produced. 

The lamp is mounted securely at its ends and connected to a source of direct current preferably 220 volts or 500 volts through suitable resistances and a switch. The switch is closed and a small Bunsen flame is played on the capillary between A and B. The mercury vaporizes and the arc strikes and is localized between the two bulbs. As soon as the lamp starts, water is turned on so that it flows down around the lamp in a large stream. The cold water keeps the cement intact and preserves the lamp. The energy is so intense that without water-cooling the quartz would be melted immediately. Hard water may be used provided that it passes over the lamp so rapidly that there is not time to heat up the water and deposit lime. The mounting of the lamp may be varied for different purposes. The lamp may be placed in front of a quartz window in a copper tank which is partially drained until the lamp is started. After starting, the tank is allowed to fill to an outlet near the top thus covering the lamp. 

The mechanism of the passage of direct current tlirough a conductor of the second class which is connected with a migration of ions and chemical reactions on electrodes can be most easily explained by an example. Let us imagine a vessel with two indifferent platinum electrodes A and C electrically connected to a source of direct current B and immersed into an aqueous solution of common salt in water (Fig. 1.). 

A battery is a galvanic cell or, more commonly, a group of galvanic cells connected in series, where the potentials of the individual cells add to give the total battery potential. Batteries are a source of direct current and have become an essential source of portable power in our society. In this section we examine the most common types of batteries. Some new batteries currently being developed are described at the end of the chapter. 

In some electrochemical cells nompontaneous chemical reactions are forced to occur by the input of electrical energy. This process is called electrolysis. An electrolytic cell consists of a container for the reaction material with electrodes immersed in the reaction material and connected to a source of direct current. Inert electrodes are often used so that they do not react. 

Solid sodium chloride does not conduct electricity. Its ions vibrate about fixed positions, but they are not free to move throughout the crystal. Molten (melted) NaCl, however, is an excellent conductor because its ions are freely mobile. Consider a cell in which a source of direct current is connected by wires to two inert graphite electrodes (Figure 21-2 a). They are immersed in a container of molten sodium chloride. When the current flows, we observe the following. 

Electroplating is performed by using a source of direct current, (a) Why can t an alternating current be used (b) What would happen if an alternating current were used for electroplating ... 

A battery consists of a galvanic cell or group of cells connected in series that serve as a source of direct current... 

The two electrodes are connected to a source of direct current capable of supplying a total of up to 180 amperes to the cell. The current is adjustable by varying the voltage of the power supply. 

An interesting variant of delay action by means of corrosion is the controlled removal of metal by anodic corrosion until a certain mechanical effect such as a spring release has taken place. The device requires a source of direct current that can be furnished by a so-called sea-water battei, as in the case of arming a mine following its placing it in the ocean, claimed by F. F. Farnsworth et al. ... 

What happens when an electric current is passed through a solution Let s consider a hydrochloric acid solution in a simple electrolytic cell, as shown in Figure 17.4. The cell consists of a source of direct current (a battery) coimected to two electrodes that are immersed in a solution of hydrochloric acid. The negative electrode is called the cathode because cations are attracted to it. The positive electrode is called the anode because anions are attracted to it. The cathode is attached to the negative pole and the anode to the positive pole of the battery. The battery supplies electrons to the cathode. 

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