Mirror reading
A mirror reading is the use of flat mirrors introduced by Poggendorff and Gauß for the precise measurement of small angles, for example the deflection of magnets by electrical current or by geomagnetic influences (see galvanometer and magnetometer ). The magnet is rotatably suspended in a horizontal plane on a cocoon thread; a small mirror is fixed above its center, perpendicular to its axis. In the mirror, through a telescope placed opposite it, one sees the mirror image of a horizontal scale, divided into millimeters, attached above the telescope, namely the middle graduation of the scale in the middle of the field of view (at the crosshairs of the telescope) when the axis of the magnet is with the reticle line (axis) of the telescope coincides. If the magnet deviates a little from this position, a different graduation line appears on the crosshairs. The small angle by which the magnet has deviated from its original position can now be determined very precisely from the distance thus read on the scale and the distance of the scale from the mirror.
The accuracy of weighing is considerably increased if a small mirror is attached to the center of the balance beam perpendicular to its length and the mirror image of a vertical scale attached next to it is observed through a side telescope (Jolly), whereby very small deflection angles can be measured. Mirror reading is also used in numerous other cases where the measurement is very small (for example, to determine expansion coefficients ).
source
- Meyers Großes Konversations-Lexikon, Volume 18. Leipzig 1909, p. 732.