  Context. Systematic observations of magnetic field strength and polarity in
sunspots began at Mount Wilson Observatory (MWO), USA in early 1917. Except for
a few brief interruptions, this historical dataset continues till present.
Aims. The sunspot field strength and polarity observations are critical in our
project of reconstructing the solar magnetic field over the last hundred years.
Here we provide a detailed description of the newly digitized dataset of
drawings of sunspot magnetic field observations. Methods. The digitization of
MWO drawings is based on a software package develope d by us. It includes a
semi-automatic selection of solar limbs and other features of the drawing, and
a manual entry of the time of observations, the measured field strength and
other notes hand-written on each drawing. The data are preserved in a MySQL
database. Results. We provide a brief history of the project and describe the
results from digitizing this historical dataset. We also provide a summary of
the final dataset, and describe its known limitations. Finally, we compare the
sunspot magnetic field measurements with other instruments, and demonstrate
that, if needed, the dataset could be continued using modern observations such
as, for example, Vector Stokes Magnetograph (VSM) on Synoptic Optical Long-term
Investigations of the Sun (SOLIS) platform.
