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Abstract
This thesis describes two pieces of apparatus designed to look for a composition-dependent intermediate-range force. Both instruments employ variants of the torsion balance technique. The first measures static torques by means of a capacitance bridge detection and feedback system. The second employs a dynamic technique which looks for shifts in the period of a torsion pendulum. Results from the static experiment, obtained at a site with a significant slope in the terrain, have been used to place limits on couplings to baryon number, isospin, and linear combinations of the two. For a coupling to baryon number, we find the product of the strength, $\alpha\sb0,$ and the range, $\lambda,$ of the interaction to be $\alpha\sb0\lambda=-0.04\pm 0.07$ m for $25<\lambda<400$ m growing linearly to $\alpha\sb0\lambda=-0.05\pm 0.09$ m at $\lambda=1600$ m. For longer ranges, out to 5 km, we find $\vert\alpha\sb0\vert<10\sp{-4}.$ Constraints on a coupling to isospin are similar in magnitude, while for couplings to $B - L$ and L the limits are approximately 26 times smaller.
The dynamic apparatus has not yet been used to look for new interactions. Non-linear behavior was found to be a major source of systematic error. This behavior may have contributed to one of the early positive results in this field for which no explanation has been found. The behavior was characterized and steps taken to minimize such effects.
The final portion of the thesis is a review of experiments looking for departures from Newtonian gravity over ranges of meters to kilometers. Current limits constrain departures from the Newtonian inverse-square law at terrestrial scales at the level of one part in 10$\sp3$ (1$\sigma).$ There is also no evidence for the existence of any composition-dependent effects at these ranges.