

Equilibrium of the Magnetic Compass in Aeroplanes. 461 



5. No abnormality in the mobility of the positive ion 

 could be observed at pressures down to 5 mm. and with very 

 large electric forces. 



6. The pressure of the electric wind in case o£ ionization 

 of both signs was studied. 



7. The recent theory of E. M. Wellisch with regard to the 

 nature of the negative ion was investigated and shown to be 

 erroneous. 



These experiments were to a great extent carried out in 

 the Physical Laboratory of the Polytechnic Institute in 

 Petrograd. I am indebted to Prof. A. F. Joffe for allowiug 

 me to work in this laboratory and to Mr. J. S. Shcheglaeff 

 for putting the necessary apparatus at my disposal. 



In its final stages the work was completed at the Cavendish 

 Laboratory, Cambridge, and I wish to take this opportunity 

 of thanking Prof. Sir J. J. Thomson for permission to use 

 the laboratory and for his interest of the progress of this 

 work. 



Cavendish Laboratory, 

 August 1916. 



XLIX. The Equilibrium of the Magnetic Compass in Aero- 

 planes. By S. G. Starling, B.Sc, West Ham Municipal 

 Technical Institute *, 



THE following investigation of the question of the 

 behaviour of the aeroplane compass was undertaken 

 at the suggestion of Mr. A. J. Hughes. It appears that the 

 deviation of the compass when under acceleration causes 

 grave errors in estimating the course, and that these differ 

 considerably from one course to another. The greatest 

 errors occur when the turn is made, as the course is changed, 

 and as these errors may arise from a variety of causes, it 

 becomes necessary to find the true position of equilibrium of 

 the compass-card, due to the magnetic effect of the earth's 

 field upon the needle. 



The calculation of the couples acting on the needle presents 

 great difficulties, as these depend upon the position of the 

 needle at every instant, but if the equilibrium position at 

 each instant is found, it can then be seen to what extent it 

 would be disturbed, and this has led to suggestions which 

 would cause the equilibrium position to be maintained true 

 magnetic N. and S. 



There is, of course, a tilting of the compass-card due to 



* Communicated by Prof. A. W. Porter, F.I7.S. 



