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shown at Fig. 45 B. This is known as the Rollinson electrolytic rectifier, which is based upon the following principles: When an element of aluminum and a corresponding element or plate of iron are submerged in a solution of certain salts, using these elements as negative and positive terminals, respectively, the passage of an

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Fig. 46.—Mercury Rectifier Bulbs and Methods of Wiring to Charge

Storage Battery From Alternating-Current Mains.

electric current through the solution produces a chemical action which forms hydroxide of aluminum. A film of hydroxide thus formed on the aluminum element repels the current. The arrangement of the cell will then permit current to pass through it in one direction only, the film of chemical preventing it from passing in the opposite direction. The result is that if an alternating cụrrent is supplied to the cell a direct pulsating current can be obtained from it. The outfits usually include a transformer for reducing the line voltage to the lower voltages needed for batterycharging purposes. Regulation of the current is effected in the simplest type by immersing the elements more or less in the solution in the jar. As complete instructions are furnished by the manufacturers, it will not be necessary to consider this form of rectifier in detail.

One of the most commonly used rectifying means is the mercury arc bulb. This device is a large glass tube of peculiar shape, as shown at Fig. 46, which contains a quantity of mercury in the base. On either side of this lower portion two arms of the glass bulbs extend outwardly, these being formed at their extremities into graphite terminals or anodes, indicated as A and A-1 in the diagram at Fig. 47. The current from the auto transformer is then attached one to each side. The base forms the cathode or mercury terminal for the negative wires. The theory of this action is somewhat complicated, but may be explained simply without going too much into detail. The interior of the tube is in a condition of partial vacuum, and while the mercury is in a state of excitation a vapor is supplied. This condition can be kept up only as long as there is a current flowing toward the negative. If the direction of the current be reversed so that the formerly negative pole becomes a positive the current ceases to flow, as in order to pass in the opposite direction it would require the formation of a new cathode element. Therefore the flow is always toward one electrode, which is kept excited by it. A A tube of this nature would cease to operate on alternating-current voltage after half a cycle if some means were not provided to maintain a flow continuously toward the negative electrode. In the General Electric rectifier tube there are two anodes and one cathode. Each of the former is connected to a separate side of the alternating current supply and also through reactances to one side of the load and the cathode to the other. As the current alternates, first one anode and then the other becomes positive, and there is a continuous flow toward the mercury cathode, thence

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Fig. 47.-Wiring Diagram Defining Use of Mercury Arc Rectifiers.

through the load in this case the battery to be charged), and back to the opposite side of the supply through a reactance. At each reversal the latter discharges, thus maintaining the are until the voltage reaches the value required to maintain the current against the counter E. M. F. and also reducing the fluctuations in the direct current. In this way a true continuous flow is obtained, with very small loss in transformation.

A small electrode connected to one side of the alternating circuit is used for starting the arc. A slight tilting of the tube makes a mercury bridge between the terminal and draws an arc as soon as the tube is turned to a vertical position. The ordinary form used for vehicle batteries has a maximum current capacity of 30 amperes for charging the lead plate type, and a larger form, intended for use with Edison batteries, yields up to a limit of 50 amperes. Those for charging ignition batteries will pass 5 amperes for one to charge six cells and a larger one that will pass 10 amperes

for from three to ten batteries. As is true of the electrolytic rectifier, complete instructions are furnished by the manufacturer for their use.

The Wagner device, which is shown at Fig. 43 A, operates on a new principle, and comprises a small two-coil transformer to reduce the line voltage to a low figure, the rectifier proper, which consists of a vibrating armature in connection with an electromagnet, and a resistance to limit the flow of the charging current. A meter is included as an integral part of the set for measuring the current flow. All sets are sold for use with ignition or lighting batteries of low voltage, with a lamp socket-plug and attaching cord, the idea being to utilize an ordinary lighting circuit of 110 volts A. C. The magnet and vibrating armature accomplish the rectification of the current with little loss, the action after connection to the battery which is to be charged proceeding automatically. By a simple device, the current stoppage throws the main contacts open, so the partially charged battery cannot be rapidly discharged. While the rectifiers are constructed to use 60-cycle, 110-volt alternating current, they will work at all frequencies from 57 to 63. The size made will pass three to five amperes, the voltage being sufficient to recharge a three-cell battery.

When batteries are to be charged from a direct current it is possible to use a rheostat to regulate the voltage at the terminals. The construction of a rheostat is very simple, as it consists only of a group of high-resistance coils of wire mounted in insulating material, and having suitable connections with segments on the base plate, upon which is mounted the operating arm that makes the contact. According to the manner in which these are made and wired a large resistance is introduced at first, gradually decreasing as the lever is moved over, or it may operate in the reverse fashion, a large amount of current being allowed to pass at the first contact and less as the handle progresses across the path. Rheostats should only be purchased after consulting a capable electrician, as the required resistance must be figured out from the voltage of the circuit to be used, the maximum battery

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Fig. 48.—Charging Storage Battery From Direct Current With the

Lamp-Bank Regulation.

current, the charging rate in amperes and the number of cells to be charged at one time.

By far the simplest method of charging storage batteries is by interposing a lamp-bank resistance instead of the rheostat. These are easily made by any garage mechanic and are very satisfactory for charging ignition or lighting batteries. Standard carbon lamps of the voltage of the circuit shown should be used, and the amperes needed for charging can be controlled by varying the candle power and the number of lamps used. If the lamps are to

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