Solar dial
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A Solar Dial is type of time switch used primarily for controlling lighting. They were/are primarily manufactured by Venner, Horstmann and Sangamo.
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[edit] Benefits
The benefit of a solar dial over a conventional 'on-off' timeswitch is the ability to 'track' the sunrise and sunset times for a particular latitude (which is specified when the unit is purchased). The solar dial 'adjusts' itself by a fractional amount each day thereby ensuring that street lighting is switched on and off when required throughout the year. Many dials also have an additional 'part night' facility allowing for a switch-off in the middle of the night, and then back on in the morning if needed. This 'part night' option was widely adopted in the United Kingdom for streetlighting in the 1970s and 1980s in order to conserve energy. Some Solar Dial switches have a clockwork or battery 'reserve' to maintain the time accuracy in case of a power outage. If this is not fitted the switch would require the labour-intensive task of being reset every time the power fails. Frequently, one timeswitch with a heavier switch rating would be used to control a whole series of lighting columns, perhaps one side of the street and another controlling the opposite side. Many columns were fitted with individual clocks though, especially on alleyways, pathways and areas in which a single column stood alone. Sometimes the timeswitch would be housed in a box fitted to a wall or telegraph pole, and the lanterns powered/switched by an extra (fifth) core on the overhead cables.
[edit] Obsolescence
These days the solar dial timeswitch has largely been replaced by 'photocell' control, these being cheaper and having less need for maintenance. solar dials are still used for situations such as stairwell lighting and car parks; and in some cases certain local authorities may request them for street lighting (though this is rare). Solar dials are often found in the rural United Kingdom, but as these fail they are invariably replaced by a photocell, usually on a new lantern and sometimes a whole new column. More recently, digital sunrise/sunset tracking timeswitches have appeared on the market, but these are generally too expensive for large scale use in street lighting and have not been adopted for this purpose, except in a few rare instances.
Although they are mostly obsolete, Sangamo still manufacture solar dials. Old examples from the 1950s, 1960s and 1970s are becoming quite collectable by some enthusiasts and are increasingly hard to find in good working condition.
