SELENIUM COMPASS. The increasing use of steel in aeroplane structure and equipment has led to the need for a compass that may be fixed in a part of the ship unaffected by magnetic disturbances. The problem, simple in principle, offers difficulties in practice. The selenium compass presents an avenue of escape, though at the expense of additional weight and complication. Selenium possesses the curious property of chang ing its electric resistance when light strikes upon it. In the selenium compass, two diametrically opposed lamps, fitted in the bottom of the compass bowl, provide two beams of light which strike upon two selenium cells incorporated in a bridge fixed on the top of the compass bowl. The magnetic system carries a diaphragm which, in one orientation of the bowl and bridge with respect to the magnetic system, will hide both beams of light from the selenium cells. If, however, the bowl be turned, the electric resistances of the two selenium cells become unequal, a condition which can be deter mined in the usual manner by means of a galvanometer fitted on the dashboard. The compass is operated in the following manner :
An instrument fitted in the cockpit has a dial marked in degrees, like an ordinary compass card. By turning a handle, the pilot or navigator can move a pointer over the dial to any desired compass course, at the same time rotating through gearing the compass bowl and bridge in the tail of the aircraft. If, for example, the handle be turned until the pointer indicates a south-east direction, then the bowl will automatically be turned so that only on this course will the diaphragm on the magnetic system equally shield the selenium cells. The pilot has thus simply to turn the aircraft until the galvanometer reading is zero ; any departure from the course will be indicated by a deflection of the galvanometer. The selenium compass has been successfully operated in flight, but, since it employs a null method of reading, it cannot be used to indicate bearings.