620 CONSTANTS OF MAGNETISATION. 



The principal term of the expression for the vertical component 

 of the action of the magnet M x on the pole + m. 2 of the bar M 2 is 

 (1146) 



Mtf*., MoL R 



The value of the vertical action on the pole - w 2 is the same ; 

 for the signs of m and of Y must be changed simultaneously. If 

 27/z 2 L 2 is replaced by M 2 , and the distance D by N/R' + I^, we see 



that 3 -! -^ is the principal term of the resultant action. 



The action of the magnet M 2 on the magnet M x is equal and 

 opposite, but it takes place on the other side, and tends to incline 

 the beam in the same direction. 



The action again changes sign when one of the bars is inverted. 

 If P 1>2 is the difference of the tares necessary for restoring equi- 

 librium in the two cases, we may write 



1VLM, 

 Pi. 2 '= 12 T" ('+/)> 



and we should determine the correction p by working at two 

 different distances. All defects of symmetry are eliminated by re- 

 versing the magnets and interchanging the pans. Here again only 

 the differences of the magnetisations by the earth comes into play. 

 With three different bars, experiment will give the products 

 MjM 2 , MjM 3 , and M 2 M 3 . Apart from the term of correction, we 

 get the equations 



M 2 P 2 . 3 ' M 3 P 2 . 3 ' M 3 P 13 ' 



which also give the moment of each bar; for we have 



M8 _(M 1 M 8 )(M 1 M S ) = RV 



' 



M 2 M 3 12 P 2 . 3 



1193. MEASUREMENT OF THE MOMENT BY THE FIELD. In 

 this experiment of Von Helmholtz the magnetic moment of a bar 

 is determined by the measurement of its field at a certain distance. 



