Quote:
Originally Posted by broadgage
The early photocells used as much power as a time switch, but only in daylight, they consumed almost nothing at night.
The lamp was switched by the contacts of a thermal relay the heater of which was wired in series with a photocell.
In daylight the cell had a low resistance and most of the mains voltage would be across the heater of the thermal relay which would absorb a watt or so and keep the contacts open.
In darkness, the cell had a high resistance, passed virtually no current, and the relay cooled down, and thus closed the contacts and lit the lamp.
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You are right about those photo bubbles on lamp heads.
I found one and can show what they are made of and how well made they were.