The original Electro 35 was introduced in 1966. It has a "cold" accessory shoe and the meter accepted film speeds from 12 to 400 ASA. Light levels are measured using a cadmium sulphide (CdS) photoresistor and powered by a mercury battery. The film speed adjustment is not implemented electronically; instead, a simple twin-bladed diaphragm closes in front of the light sensor as the film speed is reduced. The light metering electronics works by accumulating the measured light level and only releasing the shutter when it has determined enough light has fallen on the film. This system allows the shutter speed to be completely step-less and to adapt to changing light levels. SLRs would wait many years for a similar capability with off-the-film metering. The metering system can keep the shutter open for up to 30 seconds. Without a battery to power the meter, the shutter defaults to its top speed of 1/500 second.