Development of a torsion balance for measuring charging noise

Noise due to surface charge on gravitational wave detector test masses could potentially become a limiting low frequency noise source in future detectors. It is therefore very important that the behavior of charging noise is experimentally verified so that accurate predictions of charging noise can be made. A torsion balance that is sensitive to small forces has been constructed at the University of Glasgow in order to measure charging noise. In this article the torsion balance apparatus being developed will be described in detail. There will also be a description of the calibration of the instrument and preliminary measurements that have been taken. These measurements show that it is possible to distinguish between the surface charge and polarisation charge on a silica sample. From this measurement it was possible to estimate the surface charge on the silica disc. The remainder of the article will discuss the improvements in sensitivity that have been made which will allow initial measurements of charging noise to begin.