In the early 20th century, as electric power ramped up in cities and began to penetrate parts of the countryside, two types of lights competed for market share. In Circe Bloom stands in the street under the older type, a "bright arclamp." The newer incandescent bulbs, which eventually prevailed, show up in Nestor when Stephen remembers reading "under glowlamps" in the Sainte Geneviève library in Paris. Ithaca mentions both kinds: "the light of arc and glowlamps."

In the first decade of the 19th century Sir Humphry Davy, an English inventor, developed a workable electric light in which current arced across the gap between two carbon rods. (Davy originally called his invention the "arch lamp," because the flow of superheated air around the current caused it to bend.) Arc lamps were noisy and dangerous, but they emitted a harsh, bright light that could illuminate city streets, and they were cheaper than gaslights. Paris and London began to install them in 1878, and Dublin followed in 1880, building a succession of bigger power stations to feed the growing number of electric streetlights, and perhaps some indoor lights as well. (Indoor arc lights caused a number of fires in European buildings in the late 19th century.)

In 1880 Thomas Edison and Joseph Swann received a patent for their incandescent lamp, which used electric current to heat a carbon filament inside a vacuum bulb. Other inventors had been experimenting with incandescent bulbs since the 1830s, but Edison's design proved efficient and affordable. Eventually, better designs employing tungsten filaments made incandescent bulbs an attractive choice for streetlights, but arc lamps produced much more light, so they remained the choice in many cities' streetlights well into the 20th century. Today, sodium- and mercury-vapor lights, both of them types of arc lamps, are often used in streetlights.

The discussion between Stephen and Bloom in Ithaca about "the influence of gaslight or the light of arc and glowlamps on the growth of adjoining paraheliotropic trees" suggests that both types of electric streetlight could be found in Dublin in 1904. Indoors, the primary competition for incandescent bulbs was gas. Slote, Mamigonian, and Turner note that in the first years of the 20th century the "glowlamps" of the reading room of the Sainte Geneviève library mentioned in Nestor were actually "gas lamps, not electric."

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