Maker's Name: 
Cambridge Scientific Instrument Company, s/n 16979
Where made: 
16 × 16 × 30 cm

A light, two bladed aluminium vane is suspended in the center of quadrant shaped boxes, the opposite pairs of which are connected to the same potential on terminals under the base. The vane is attached to a light rod which carries a small mirror suspended by a thin quartz fibre (missing) from the torsion head. A brass case with window opposite the mirror completes the apparatus. The quadrant electrometer may be used to compare the EMF of two cells; verify Ohm's Law; measure a high resistance; compare large and small capacitances and determine the dielectric constant. This is done by deflection of the aluminium vane via a difference in the electric potential of the pairs of quadrants. The angle of deflection is measured by shining a lamp on the mirror, which reflects the light onto a scale placed a certain distance away. The instrument must be calibrated so that the scale used is accurate and its sensitivity known but it draws no current from the circuit and is sensitive to a fraction of a volt.  Serial Number 16979.