The axial attmctiou is — , if a leptesents half the length of the 



iiiterferricum. If we integrate along h between the limits a and 

 V^a" 4- q\ then we find 



a 

 Ho-^ 1 — — — =1 — cos a 



Hence : i7, -\-H^ -- 1 — cos « -\- sin a cos « Ign — . 



Q 

 The maximal valne of this expression depends on the magnitude 



R:Q. \i Q is infinitely small, which means that the side-planes of 



the prism terminate in the natural rib, we find again a = 45°. 



With other values of R we find : 



In the string- galvanometer, as a rule, R-.q will be somewhere 

 between 25 and 50. By making the top-angle about 51° we obtain 

 the maximal field-intensity. As however the value of the expression 

 for B^ -\- H^ does not vary much in the neighbourhood of the maxi- 

 mum, a little deviation in size of the angle will have no prejudicial 

 consequences. 



In general we can say that it is better to make the angle a little 

 larger than the theoretical value, as in that case the field will 

 become more homogeneous, whilst with a smaller angle the field- 

 intensity will diminish more rapidly towards the rims. As a maximum 

 perhaps 53° — -55° may be taken ; i.e. with a core of 5 cm. diameter 

 the angle should be nearly 55° and with a core of 10 cm. 53° must 

 not be surpassed. 



11. The magnetic field and the shape of the strim/ in the ^I'STRovmi' 

 galvanometer. 



As soon as a constant current passes through the string of the 

 galvanometer of Einthoven the string assumes a curved shape. If 

 we wished to draw this shape, we might use a well-known grapho- 

 statical method of construction. It is used e.g. to draw the shape 

 of a chain of a suspension bridge. 



Suppose the chain is suspended between A and B and the loads 



34=^ 



