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

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 arc 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

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

operate on 110-volt circuit, a 16-candle-power carbon filament lamp will permit one-half ampere to pass; to 32-candle-power will allow 1 ampere to pass. If it is desired, therefore, to pass three amperes through the battery, one could use 3 32-candle-power lamps, or 6 16-candle-power lamps. If the lamps are to burn on 220 volts, it should be remembered that when the voltage is doubled the amperage is cut in half, therefore the 32-candlepower, 220-volt carbon filament bulbs will only pass half an ampere. The method of wiring is very simple, as may be readily ascertained by referring to Fig. 48. The line wires are attached to a fuse block and then to a double knife switch. The switch and fuse block are usually mounted on a panel of insulating material such as slate or marble. One of the wires, the positive of the circuit, runs from the switch directly to the positive terminal of the storage battery. The negative wire from the switch passes to the lamp-bank resistance. The lamps are placed in parallel connection with respect to each other, but in series connection in respect to the battery. When coupled in this manner the current must overcome the combined resistance of the storage battery, which is very low, and that of the lamps. This prevents the battery being charged with current of too high voltage.

A water resistance is easily constructed by using a small wooden tub or half barrel. Two sheet-lead plates are suspended from wood sticks resting on top of the tub, the supports being movable to bring the lead plates closer together or separate them, as desired. A wire is brought from the battery, as shown in Fig. 49, to one side of the switch, then to one of the plates in the water resistance, then from the other plate of the water resistance to an ammeter; to the other side of the switch and from there to the opposite pole of the battery. Such a resistance is used for making a test discharge of a vehicle battery; it would not be a very practical way of charging batteries because of the great absorption of current by the water. Before starting a discharge, care should be taken to have the lead plates and wires separated. The tub can then be filled with clean water and the switch closed. A small quantity of electrolyte should then be poured in the water, a very little at a time, until the ammeter shows that the proper

amount of current is flowing. The farther apart the plates are, the greater the resistance. As more electrolyte is added, even if the plates are not disturbed, the resistance becomes less. Never let the plates touch each other.