Keith Lucas. 



xxxvii 



overcome was to find oat why the error was capricious ; sometimes it showed 

 itself and sometimes it did not, and it became clear that although it depended 

 on the rate of the turn, it also depended on something else. After much 

 flying and observation of a compass in the air, he found that this error was 

 very great if a deviation was made when the direction of flight was towards 

 the north. In this case the compass needle was so far carried round with 

 the aeroplane, when on a turn, that the flier might think he was flying in a 

 straight line, although he was turning somewhat rapidly. In a cloud, or at 

 night when there were no visible objects to act as guides, this was a great 

 danger. If the aeroplane was flying in a southerly direction the compass 

 needle turned in the opposite direction, and the flier would get an exaggerated 

 estimate of his rate of turning. As the flier's object is to fly straight 

 through a cloud this would not matter. 



The abnormal behaviour of the standard compass in the air was utterly 

 unexpected, and the value of the discovery was great. In order to remedy 

 this defect, which was found in all existing compasses, much experimental 

 work had to be done in the air ; he formed tlieories of the cause of the 

 error, tested them in the air, and after eliminating those which proved 

 wrong, at last found the true cause. 



The magnetic forces act on the poles of the magnet in the direction of the 

 dip, and tend to rotate it in a vertical plane. When the aeroplane is flying 

 straight this tendency is balanced by displacing the centre of gravity 

 towards the south pole, in order to keep the magnet horizontal. But when 

 the aeroplane is turning the apparent direction of gravity is no longer the 

 true vertical : the magnetic forces, however, still act in a vertical plane, as 

 before, and this change of conditions produces the turning error. 



The compass that Lucas made was a great improvement on the existing 

 patterns. Its special features are the combination of the antivibration 

 mounting ; the spherical bowl to contain the liquid ; a magnetic system 

 small in relation to the size of the bowl, with a long period of vibration ; 

 graduations on a short cylinder instead of a disc ; and the inverted pivot. 



Unknown to Lucas, most or all of these features had been tried before 

 for marine compasses ; but they are not called for at sea, owing to the 

 relatively slow speed of ships, and had been long forgotten. By his work 

 he brought them into use for aeroplane compasses, and they are an important 

 life-saving factor. 



One of the reasons wliy vibration causes errors in compasses was pointed 

 out by Mr. A. Mallock to the Advisory Committee for Aeronautics, and it 

 fell to me to be of some little assistance to Lucas with regard to this error. 

 Theoretical considerations showed that this vibration error would be reduced 

 by inverting the usual arrangement in compasses in which an agate cup is 

 carried by the card and rests on a needle-point fixed to the compass bowl. 

 If the needle-point is fixed to the magnetic system and the agate cup is 

 supported by the compass bowl the vibration errors were reduced. Lucas 

 looked into the theory of the vibration error, confirmed the experiments I 



