A metal bridge is
mounted on a tripod stand and a metal pillar. The bridge has vertical end
pieces between which is stretched a fine steel torsion wire. The torsion
wire held in a chuck can be turned by a milled brass knob. A metal pointer
is attached firmly to the wire at right angles. The pointer moves over a
scale attached to the metal pillar and carries an adjustable weight at the
other end. A scale pan with a hook is hung from the pointer and carries a
wire frame from the hook.