(556 Professor W. G. Adams [June 3, 



WEEKLY EVENING MEETING, 



Friday, June 3, 1881. 



Thomas Boycott, M.D. F.L.S. Vice-President, in the Chair. 



Professor W. G. Adams, M.A. F.E S. 



Magnetic Distwhance, Aurorce, and Earth Currents. 



The object of establishing a magnetic observatory is to determine at 

 any instant the direction and magnitude of the earth's magnetic 

 force. 



The direction of the magnetic force of the earth is the direction 

 in which a small magnetic needle would point when it is freely 

 suspended, so as to turn about an axis passing through its centre of 

 gravity. But it is not easy to suspend a magnetic needle so as to 

 turn freely, and yet to be sure that the axis about which it turns 

 passes accurately through the centre of gravity of the needle ; and if 

 it does not so pass, then on suspending the needle we have not only 

 the magnetic force, but also the gravitating force of the earth acting 

 upon it to turn it about its axis, and the position which it takes up 

 shows us the direction of these combined forces upon the magnetic 

 needle. 



This direction depends upon the mass of the needle, for to that its 

 weight is due; it depends upon the form of the needle, and the 

 posftion of its centre of gravity with regard to the axis on which it is 

 hung ; it depends also on the magnetic properties of the substance, 

 so that it is not easy to determine even the direction of the magnetic 

 force by a plan which, theoretically, is so very simple. 



Instead of attempting to make the required determinations by 

 such a method, it is necessary that a steadier mode of suspension 

 should be adopted, and that may be done as soon as it is discovered 

 in what vertical plane the force of gravity, combined with the earth's 

 magnetic force, will cause such a needle to rest. 



This is usually done by loading a steel needle at one end, and 

 then magnetising it with its poles so arranged that the extra w^eight 

 of the heavier end shall balance the downward pull of the magnetic 

 force on the other end. In this case the needle when magnetised 

 will remain at rest in a horizontal direction when suspended on a 

 point on which it can turn freely in a horizontal plane. 



A magnetic needle suspended in this way has been called a 

 declination needle. Such a needle is employed in the mariner's 

 compass, in our galvanometers for measuring currents of electricity, 

 and in magnetic observatories for determining the declination, or 

 what is sometimes called the variation, of the magnetic needle. This 



