Part 17

About 1900 the constant current transformer was developed by Elihu Thomson. This transforms current taken from a constant potential alternating current circuit into a constant alternating current for series circuits, whose voltage varies with the load on the circuit. The transformer has two separate coils; the primary being stationary and connected to the constant potential circuit and the secondary being movable and connected to the series circuit. The weight of the secondary coil is slightly underbalanced by a counter weight. Current flowing in the primary induces current in the secondary, the two coils repelling each other. The strength of the repelling force depends upon the amount of current flowing in the two coils. The core of the transformer is so designed that the central part, which the two coils surround, is magnetically more powerful close to the primary coil than it is further away.

When the two coils are close together a higher voltage is induced in the secondary than if the later were further away from the primary coil. In starting, the two coils are pulled apart by hand to prevent too large a current flowing in the series circuit. The secondary coil is allowed to gradually fall and will come to rest at a point where the voltage induced in it produces the normal current in the series circuit, the repelling force between the two coils holding the secondary at this point. Should the load in the series circuit change for any reason, the current in the series circuit would also change, thus changing the force repelling the two coils. The secondary would therefore move until the current in the series circuit again becomes normal. The action is therefore automatic, and the actual current in the series circuit can be adjusted within limits to the desired amount, by varying the counterweight. A dash pot is used to prevent the secondary coil from oscillating (pumping) too much.

In the constant current transformer, the series circuit is insulated from the constant potential circuit. This has many advantages. A similar device, called an automatic regulating reactance was developed which is slightly less expensive, but it does not have the advantage of insulating the two circuits from each other.

ENCLOSED SERIES ALTERNATING CURRENT ARC LAMPS

The simplicity of the constant current transformer soon drove the constant direct-current dynamo from the market. An enclosed arc lamp was therefore developed for use on alternating constant current. Two sizes of lamps were made; one for 6.6 amperes consuming 450 watts and having an efficiency of about 4½ lumens per watt, and the other 7.5 amperes, 480 watts and 5 l-p-w (clear glassware). These lamps soon superseded the direct current series arcs. They have now been superseded by the more efficient magnetite arc and tungsten filament incandescent lamps.

SERIES INCANDESCENT LAMPS ON CONSTANT CURRENT TRANSFORMERS

Series incandescent lamps were made for use on constant current transformers superseding the “Municipal” and “Shunt Box” systems. The large Edison, now called the Mogul Screw base, was adopted and the short circuiting film cut-out was removed from the base and placed between prongs attached to the socket.

Holder. Socket. Holder and socket.

SERIES INCANDESCENT LAMP SOCKET WITH FILM CUTOUT, 1900.

The “Large Edison,” now called Mogul Screw, base was standardized and the short circuiting device put on the socket terminals.]

The transformers made for the two sizes of arc lamps, produced 6.6 and 7.5 amperes and incandescent lamps, in various sizes from 16 to 50 cp, were made for these currents so that the incandescent lamps could be operated on the same circuit with the arc lamps. The carbon series incandescent lamp, however, was more efficient if made for lower currents, so 3½-, 4- and 5½-ampere constant current transformers were made for incandescent lamps designed for these amperes. Later, however, with the advent of the tungsten filament, the 6.6-ampere series tungsten lamp was made the standard, as it was slightly more efficient than the lower current lamps, and was made in sizes from 32 to 400 cp. When the more efficient gas-filled tungsten lamps were developed, the sizes were further increased; the standard 6.6-ampere lamps now made are from 60 to 2500 cp.

THE NERNST LAMP

Dr. Walther Nernst, of Germany, investigating the rare earths used in the Welsbach mantle, developed an electric lamp having a burner, or “glower” as it was called, consisting of a mixture of these oxides. The main ingredient was zirconia, and the glower operated in the open air. It is a non-conductor when cold, so had to be heated before current would flow through it. This was accomplished by an electric heating coil, made of platinum wire, located just above the glower. As the glower became heated and current flowed through it, the heater was automatically disconnected by an electro-magnet cut-out.

The burners consisted mainly of zirconium oxide which had to be heated before current could go through them.]

The resistance of the glower decreases with increase in current, so a steadying resistance was put in series with it. This consisted of an iron wire mounted in a bulb filled with hydrogen gas and was called a “ballast.” Iron has the property of increasing in resistance with increase in current flowing through it, this increase being very marked between certain temperatures at which the ballast was operated. The lamp was put on the American market in 1900 for use on 220-volt alternating current circuits. The glower consumed 0.4 ampere. One, two, three, four and six glower lamps were made, consuming 88, 196, 274, 392 and 528 watts respectively. As most of the light is thrown downward, their light output was generally given in mean lower hemispherical candlepower. The multiple glower lamps were more efficient than the single glower, owing to the heat radiated from one glower to another. Their efficiencies, depending on the size, were from about 3½ to 5 lumens per watt, and their average candlepower throughout life was about 80 per cent of initial. The lamp disappeared from the market about 1912.

THE COOPER-HEWITT LAMP

In 1860 Way discovered that if an electric circuit was opened between mercury contacts a brilliant greenish colored arc was produced. Mercury was an expensive metal and as the carbon arc seemed to give the most desirable results, nothing further was done for many years until Dr. Peter Cooper Hewitt, an American, began experimenting with it. He finally produced an arc in vacuum in a one-inch glass tube about 50 inches long for 110 volts direct current circuits, which was commercialized in 1901. The tube hangs at about 15 degrees from the horizontal. The lower end contains a small quantity of mercury. The terminals are at each end of the tube, and the arc was first started by tilting the tube by hand so that a thin stream of mercury bridged the two terminals. Current immediately vaporized the mercury, starting the arc. A resistance is put in series with the arc to maintain the current constant on direct current constant voltage circuits. Automatic starting devices were later developed, one of which consisted of an electro-magnet that tilted the lamp, and the other of an induction coil giving a high voltage which, in discharging, started the arc.

This gives a very efficient light, practically devoid of red but of high actinic value, so useful in photography.]

This lamp is particularly useful in photography on account of the high actinic value of its light. Its light is very diffused and is practically devoid of red rays, so that red objects appear black in its light. The lamp consumes 3½ amperes at 110 volts direct current (385 watts) having an efficiency of about 12½ lumens per watt.

The mercury arc is peculiar in that it acts as an electric valve tending to let current flow through it only in one direction. Thus on alternating current, the current impulses will readily go through it in one direction, but the arc will go out in the other half cycle unless means are taken to prevent this. This is accomplished by having two terminals at one end of the tube, which are connected to choke coils, which in turn are connected to a single coil (auto) transformer. The alternating current supply mains are connected to wires tapping different parts of the coil of the auto transformer. The center of the coil of the auto transformer is connected through an induction coil to the other end of the tube. By this means the alternating current impulses are sent through the tube in one direction, one half cycle from one of the pairs of terminals of the tube, the other half cycle from the other terminal. Thus pulsating direct current, kept constant by the induction coil, flows through the tube, the pulsations overlapping each other by the magnetic action of the choke coils. This alternating current lamp is started by the high voltage discharge method. It has a 50-inch length of tube, consuming about 400 watts on 110 volts. Its efficiency is a little less than that of the direct current lamp.

The mercury arc is inherently for use on direct current, but by means of reactance coils, it can be operated on alternating current.]

THE LUMINOUS OR MAGNETITE ARC LAMP

About 1901 Dr. Charles P. Steinmetz, Schenectady, N. Y., studied the effect of metallic salts in the arc flame. Dr. Willis R. Whitney, also of Schenectady, and director of the research laboratory of the organization of which Dr. Steinmetz is the consulting engineer, followed with some further work along this line. The results of this work were incorporated in a commercial lamp called the magnetite arc lamp, through the efforts of C. A. B. Halvorson, Jr., at Lynn, Mass. The negative electrode consists of a pulverized mixture of magnetite (a variety of iron ore) and other substances packed tightly in an iron tube. The positive electrode is a piece of copper sheathed in iron to prevent oxidization of the copper. The arc flame gives a brilliant white light, and, similar to the mercury arc, is inherently limited to direct current. It burns in the open air at about 75 volts. The lamp is made for 4-ampere direct current series circuits and consumes about 310 watts and has an efficiency of about 11½ lumens per watt.

This has a negative electrode containing magnetite which produces a very luminous white flame in the arc stream.]

The negative (iron tube) electrode now has a life of about 350 hours. Later, a higher efficiency, 4-ampere electrode was made which has a shorter life but gives an efficiency of about 17 l-p-w, and a 6.6-ampere lamp was also made giving an efficiency of about 18 l-p-w using the regular electrode. This electrode in being consumed gives off fumes, so the lamp has a chimney through its body to carry them off. Some of the fumes condense, leaving a fine powder, iron oxide, in the form of rust. The consumption of the positive (copper) electrode is very slow, which is opposite to that of carbon arc lamps on direct current. The arc flame is brightest near the negative (iron tube) electrode and decreases in brilliancy and volume as it nears the positive (copper) electrode.

The method of control, entirely different from that of other arc lamps, was invented by Halvorson to meet the peculiarities of this arc.]

The peculiarities of the arc are such that Halvorson invented an entirely new principle of control. The electrodes are normally apart. In starting, they are drawn together by a starting magnet with sufficient force to dislodge the slag which forms on the negative electrode and which becomes an insulator when cold. Current then flows through the electrodes and through a series magnet which pulls up a solenoid breaking the circuit through the starting magnet. This allows the lower electrode to fall a fixed distance, about seven-eighths of an inch, drawing the arc, whose voltage is then about 72 volts. As the negative electrode is consumed, the length and voltage of the arc increases when a magnet, in shunt with the arc, becomes sufficiently energized to close the contacts in the circuit of the starting magnet causing the electrode to pick up and start off again.

MERCURY ARC RECTIFIER FOR MAGNETITE ARC LAMPS