210 Mr. B. Davies on a new form of Amperemeter 



Therefore, the total deflecting moment M acting on the 

 coil is 



H nc \( - — r j r -+ 2 \ xdx\. 



So M = £ncHJr. 



Unlike the old coil (that considered in the first part of this 

 paper) this new coil has a deflecting moment increasing 

 always in proportion to its radius if the magnetic field is 

 kept constant. The moment is therefore a maximum when r 

 is as nearly \l as the constructional details of the instrument 

 will permit. For the old coil the maximum moment is got 

 when r = ^l, i. e., when the coil is a square. I offer the 

 description of this instrument partly on account of its leading 

 to (what is to me) a novel form of a ballistic galvanometer. 

 There may be more serious difficulties in the way of con- 

 structing a galvanometer on this plan than I at present 

 anticipate. 



Ballistic Galvanometer. 



We evidently have in the arrangement shown in figs. 5, 6, 

 and 7 favourable conditions for a ballistic galvanometer, since 

 the deflecting moment and the moment of inertia of the coil 

 attain their maximum values for the same value of the radius. 

 The period of swing and the deflecting moment may there- 

 fore be increased simultaneously without making any addition 

 to the weight of the moving system. 



Let fig. 8 be a section through the polepieces of a ballistic 



Fiff. 8. 



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arrangement on the above plan, the coil completely embrac- 

 ing the central polepiece NN, and the outer polepiece S 

 completely surrounding both. Here, of course, there is to be 

 no aluminium frame for damping. 



To compare the sensibility of this coil with another coil of 

 the same mass, effective area, and number of turns, wound and 

 suspended in the usual mode, let fig. 9 represent one, and 



