Horizontal Component of the Earth's Magnetic Field. 493 



Referring to the figure, m is the magnetometer-needle ; 

 and X two coils, consisting of one layer of No. 30 B.W.G. 

 silk-covered copper wire, wound in glass tubes about 5 millim. 

 in external diameter ; C is a scale divided to millimetres, on 

 which the deflection of the magnetometer-needle is read, in 

 the manner described for the deflection-experiment above ; 

 R is a box of resistance-coils, and Gr a current-galvanometer. 

 The line D E represents a magnetic meridian, and A F a line 

 passing through the centre of m and at right angles to D E. 



The coil C was placed with its axis parallel to A F, and its 

 centre on the line DE. The coil G ± was placed with its axis 

 on the line A F, and at such a distance from m that a cur- 

 rent passed in the proper directions through both coils pro- 

 duced no deflection. In making this adjustment a current, 

 equal to about 30 times the greatest current subsequently used 

 in the experiment, was passed through the coils. 



The magnet to be tested was then introduced into one of 

 the coils, adjusted to the proper position, and the deflection of 

 m read. A current of such strength as to produce a field of 

 about ^q C.Gr.S. units intensity was then passed through the 

 coils, and the change of deflection read. The current was 

 then reversed, and the deflection again read. This operation 

 was repeated with stronger and stronger currents until a field 

 of from one to two units intensity was reached. The magnet 

 was next introduced into the other coil, and a similar series 

 of measurements taken. The results plotted on a sheet of 

 section-paper show clearly that a field of considerably greater 

 intensity than that used is required to permanently alter the 

 magnetic moment of the magnets when they are hard- 

 tempered. 



The changes of deflection obtained when the magnet is in 

 the coil are alw r ays a smaller fraction of the total deflection 

 than they are when it is in the coil G v This is no doubt due 

 to a change of magnetic distribution. The equations to the 

 deflections in the two cases are : — 



*= M 



H(Y 2 + a?) f 

 for the coil C 0j and 



2rM 

 *i- H(r 2 -a») 2 

 for the coil C v 



From these equations it is clear that either a change of M or 

 of a x w T ill affect the deflection, but that the effect of the change 

 of «i is opposite in the two cases. The method of measurement 



