Philips Historische Producten museum Eindhoven (40/52)

Here's a series of special incandescent lamps intended for signaling (left) and scientific/technical applications (center and right). The center lamp is a very peculiar one, it uses an electrical discharge in a low-pressure nitrogen-helium or argon atmosphere to heat up ball-ended tungsten electrodes to a very high temperature. The arc itself is not luminous and the primary source of light is the pair of incandescent electrodes, which forms a point-like source of light useful in optical applications.