| Summary: | The magnetic field is a major contributor to the physics of many types of astronomical objects. Polarimetry is a powerful tool for probing directly into the magnetic field to estimate its degree of ordering and orientation. This knowledge is often essential in order to constrain theoretical models in astrophysics. The advent of submillimetre polarimetry has opened a new window through which polarised emission produced by dust grains or the synchrotron mechanism can be investigated. This has immediate applications to the study of star forming regions and radio-loud extragalactic sources. The work presented in this thesis covers several aspects of doing polarimetry in the submillimetre with the James Clerk Maxwell Telescope (JCMT), and its application to an on-going observing programme dedicated to monitor the polarisation of highly variable extragalactic sources, which include BL Lac Objects and a subset of Quasars, often referred to collectively as Blazars. The more instrumental parts of this project are concerned with the development of data reduction algorithms and their implementation in a new common user software package for the analysis of JCMT polarimetry data, the optimisation of data acquisition techniques in the currently available observing mode and the development of a new (potentially more efficient) observing mode, including numerical simulations, extensive laboratory experiments and telescope tests. The Blazar monitoring programme consists of a series of observing runs during which polarimetry and multi-frequency photometry have been done on a sample of sources. The observations allow two different types of study to be carried out: one in terms of the variability of individual sources and the other in terms of the overall properties derived for the two classes of object. While variability observations are used to discern between and place constrains on theoretical models that deal with the energy production and transport mechanisms, the comparison between BL Lac Objects and Quasars investigates whether they are likely to have different parent populations, or if their properties can be accounted for within a single unifying theory.
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