1881.] OH Magnetic Disiurbann^ Aunna', dnd Earth Cnrnufs. T.;")? 



ueedlo determines the position of the verticul jJuiio in wliicli lies tlio 

 direction of the earth's mugnetic force, and which is called the piano 

 of the mafrnetic meridian. The plane of the maijnrtlc meridian is 

 usually dillerent from the vertical i)lane through the nortli and south 

 pedes, which is called the geographical meridian, or the meridian, and 

 the angle between these two planes is the declination or variation of 

 the magnetic ueedlo. 



If such a magnetic needle as I have just described be supported 

 on horizontal knife-edges instead of being supported on a point, the 

 needle when magnetised may remain at rest balanced in a horizontal 

 direction, one end being pulled downwards by the eartli's vertical 

 magnetic force, and the other by the force of gravity. Any change 

 in the intensity of the vortical magnetic force of the earth will bo 

 shown by an up or down motion of the marked end of the needle. 

 Such an instrument, culled a balance magnetometer, is specially 

 adapted for showing any changes in the vertical magnetic force of 

 the earth, and any changes or disturbances of the earth's vertical 

 magnetic force of whicli I may speak this evening have been deter- 

 mined by means of such a balance magnetometer. We have then 

 our declination or variation ucedle to determine the vertical plane 

 called the magnetic meridian, and we have our balance magnetometer 

 to determine any changes which may take place in the vertical 

 magnetic force of the earth. 



By the declination needle we can not only determine the j^lane of 

 the magnetic needle, but by making the needle oscillate to and fro 

 horizontally and counting the number of oscillations in a given time, 

 we can determine the horizontal pull ui:>on the poles of tiio needle ; 

 i. e. the intensity of the earth's horizontal magnetic force upon the 

 needle, just as by the swing of a simple i)endulum in a vertical plane 

 under the action of the force of gravity we can determine the })ull of 

 the force of gravity upon the bob of the pendulum. By a similar 

 method and by a properly suspended needle, either the vertical force 

 or the total magnetic force of the earth may be determined. 



In order then to determine the direction of the earth's magnetic 

 force, we may make use of a declination needle to give us the vertical 

 plane, and place the di2)ping needle in such a position tliat it will 

 oscillate in that plane. When it comes to rest it will point in the 

 direction of the total magnetic force, i. e. in the direction through the 

 room of Faraday's lines of magnetic force. In order to determine 

 the magnitude of that force tlie horizontal force may be found by 

 finding the number of oscillations of the declination needle in the 

 way that I have already exi^lained, and these three determinations 

 will give us the direction and magnitude of the earth's total magnetic 

 force. 



Another method of making tlie required determinations is to 

 take a coil of copper wire, which is wound on a circular frame in such 

 a way as to be capable of spinning on a diameter of the circular 

 frame. 



