EFFECT OF INDUCTION MEASUKED. 163 



regard to the viewing telescope ; the whole apparatus 

 being turned horizontally round a vertical axis till that 

 condition is obtained, and the graduated horizontal 

 circle which registers its rotation being then read. 

 But there is this difference in adjustment from that of 

 Article 26, that the magnet is placed in a vertical posi- 

 tion, with a definite point near one pole exactly in the 

 horizontal plane of the disturbed needle. 



Suppose now that the red pole of the magnet is 

 downwards, a mark near the red pole being at the same 

 level as the needle, and the blue pole projecting far 

 above the level of the needle. The effect of induc- 

 tion by the earth's vertical force is to add to the red 

 power of the lower end and to the blue power of the 

 upper end, and in fact to make the magnet more power- 

 ful. Now invert the magnet, so that the mark near the 

 red pole is still at the same level as the needle, but the 

 blue pole projects far below the level of the needle. As 

 regards the action of the magnet upon the needle, the 

 force exercised is the same as before. But to the red 

 magnetism at the upper end of the magnet there is now 

 added blue magnetism produced by the earth's vertical 

 induction, and to the blue magnetism at the lower end 

 there is added red magnetism produced by induction, 

 and the power of the magnet is diminished. And 

 these vertical magnetisms are not affected by the hori- 

 zontal rotation of the apparatus round the needle. It is 

 evident here that we have the means of determining the 

 propartion of the induced part to the permanent part of 



magnetism. 



112 



