The Campbell-Stokes sunshine recorder is one of meteorology's most elegant instruments: a glass sphere focuses sunlight onto a graduated card, burning a trace whose length indicates the day's effective sunshine hours.
Continued relevance
Although electronic sensors are replacing it, the recorder remains in use at many AEMET and WMO stations for its reliability and simplicity (no electricity or frequent calibration needed). Historical sunshine data from Spain since the 19th century comes from these recorders, making them irreplaceable for long climate series.