Context: A variation of the incandescent lamp did not use a hot wire filament, but instead used an arc struck on a spherical bead electrode to produce heat. The electrode then became incandescent, with the arc contributing little to the light produced. Such lamps were used for projection or illumination for scientific instruments such as microscopes. These arc lamps ran on relatively low voltages and incorporated tungsten filaments to start ionization within the envelope. They provided the intense concentrated light of an arc lamp but were easier to operate. Developed around 1915, these lamps were displaced by mercury and xenon arc lamps.

Question: What did an alternate version of the incandescent lamp substitute for a wire filament?
Answer: an arc struck on a spherical bead electrode

Question: What was a common application for this type of bulb?
Answer: scientific instruments such as microscopes

Question: When was this style of lamp developed?
Answer: 1915

Question: What displaced this style of lamp?
Answer: mercury and xenon arc lamps

Question: What type of lamp used a hot wire filament?
Answer: Unanswerable

Question: What type of lamp did not use an arc struck on a spherical bead electrode?
Answer: Unanswerable

Question: What was not a common use for this type of bulb?
Answer: Unanswerable

Question: What type of lamp typically ran on relatively high voltages?
Answer: Unanswerable

Question: What type of lamp provided a low concentrated light that was harder to operate?
Answer: Unanswerable