12.2 × 1.4 × 2 cm

This instrument has a window at one end to allow in radiation, mainly beta (electrons), and an eyepiece at the other end. The electrons charge a metal fibre inside the instrument. The fibre deflects proportionally to the charge, and appears in the eyepiece as a line against a graduated scale marked in milliRoentgens. The instrument is clipped in the breast pocket, and is checked regularly to indicate the radiation dose received by the user.