The mercury vapor lamp in this country is put on the market by the Cooper-Hewitt Electric Company and it is being used to a considerable extent for industrial illumination. In this mercury vapor, rendered incandescent by the passage of an electric current through it, is the source of light. In its standard form, this lamp consists of a long glass tube from which the air has been carefully exhausted, and which contains a small amount of metallic mercury. The mercury is held in a large bulb at one end of the tube and forms the negative electrode in the direct-current lamp. The other electrode is formed by an iron cup and the connections between the lamp terminals and the electrodes are of platinum where this connection passes through the glass. Fig. 28 gives the general appearance of a standard lamp having the following specifications: Total watts (110 volts, 3.5 amperes) = 385 Candle-power (M. H. with reflector) = 700 Watts per candle = 0.55 Length of tube, total = 55 in. Length of light-giving section = 45 in. Diameter of tube = 1 in.

Height from lowest point of lamp to ceiling plate = 22 in. For 220-volt service two lamps are connected in series. The mercury vapor, at the start may be formed in two ways: First, the lamp may be tipped so that a stream of mercury makes contact between the two electrodes and mercury is vaporized when the stream breaks Second, by means of a high inductance and a quick break switch, a very high voltage sufficient to pass a current from one electrode to the other through the vacuum, is induced and the conducting vapor is formed. The tilting method of starting is preferred and this tilting is brought about automatically in the more recent types of lamp. Fig. 29 shows the connections for automatically starting two lamps in series. A steadying resistance and reactance are connected as shown in this figure.

The mercury vapor lamp is constructed in rather large units, the 55-volt, 3.5-ampere lamp being the smallest standard size. The color of the light emitted is objectionable for some purposes as there is an entire absence of red rays and the light is practically monochromatic. The illumination from this type of lamp is excellent where sharp contrast or minute detail is to be brought out, and this fact has led to its introduction for such classes of lighting as silk mills and cotton mills. On account of its color the application of this lamp is limited to the lighting of shops, offices, and drafting rooms, or to display windows where the goods shown will not be changed in appearance by the color of the light. It is used to a considerable extent in photographic work on account of the actinic properties of the light.

Fig. 28. Cooper Hewitt Mercury Vapor Lamp.

Fig. 28. Cooper-Hewitt Mercury Vapor Lamp.





Colossal Statue by Bartholdt, Erected on Bedloe's Island, New York. A Gift of the French People to the American Nation. Dedicated in 1886. Total Height 306 Feet.

Height of Statue without Pedestal, 151 feet. The Statue Consists of a Trusswork of Iron Beams Enveloped in a Sheathing of Copper Plates Hammered into Shape. Over 200 Tons of Metal was Used in the Construction. The Pedestal, Designed by the Late Richard H. Hunt, was Paid for Largely by Popular Subscription in the United States.

Special reactances must be provided for a mercury arc lamp operating on single-phase, alternating-current circuits.