brand Double hairpin lamp. First introduced in 1905,
these used "GEM" filaments which were
metalised carbon that had a lower resistance than normal
carbon, and burned brighter. Since the resistance was
lower, they needed to have a longer length of filament
inside to run from the mains voltage so the Double
Hairpin design was adopted- essentially 2 identical
filaments connected in series.
breadboard mounted lamp sockets & "unmounted"
form of knife-switch with Gutta-Percha insulated