A brass case with window opposite the mirror completes the apparatus. 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. D.C. Measurements Booklet No. III pg 68.
Lever reverses two secondary coils inside two fixed primaries.
The secondary coils are spring loaded, the springs also carrying the current for the secondaries. The Duddell magnetic standard was used for the calibration of ballistic galvanometers in conditions similar to those in which they were used. A known direct current of up to 10A is passed through the fixed primary coils, and the galvanometer is connected in series with the secondary coils. Releasing the lever on the front allows the secondary coils to reverse, causing a known flux change through the secondaries, and thereby a known total charge to flow through the galvanometer. The deflection of the ballistic galvanometer is proportional to the total charge, and therefore this allows it to be calibrated.