title: Long-term studies of photospheric magnetic fields on the Sun
abstract: We briefly review the history of observations of magnetic fields on the Sun, and describe early magnetograps for full disk measurements. Changes in instruments and detectors, the cohort of observers, the knowledge base etc may result in non-uniformity of the long-term synoptic datasets. Still, such data are critical for detecting and understanding the long-term trends in solar activity. We demonstrate the value of historical data using studies of active region tilt (Joy’s law) and the evolution 
journal: Journal of Space Weather and Space Climate