This lamp, which provides some illumination for the steps leading down to Salts Mill, is of a different design from the other street lights and lamps in Saltaire. It is rather elegant, with its long slender arched neck. I suspect it is of a different period and has probably always been an electric light, whereas many of the other street lights were converted from gas lamps. The first street in the UK to be lit by incandescent electric light was Mosley Street in Newcastle-upon-Tyne in February 1879 - but I imagine this Saltaire light dates perhaps from the 1920s or 30s (I don't know for sure).
Whilst we're on the subject of lamps, did you know that there is still a Victorian street lamp in London that is powered by sewer gas? I don't think the Saltaire lights were like this but it's a neat idea for our own eco-conscious times, perhaps?
You can get a sense from this photo of the solid, dark bulk of the mill buildings as the night draws in. I always fancy that the mill sits 'on guard' over its village through the night.